diff --git a/ltm/include/ltm/common.h b/ltm/include/ltm/common.h
index ef86779..6851359 100644
--- a/ltm/include/ltm/common.h
+++ b/ltm/include/ltm/common.h
@@ -17,10 +17,16 @@
     }                                                                          \
   }
 
+#define LTM_MIN(x, y) ((x) < (y) ? (x) : (y))
+#define LTM_MAX(x, y) ((x) > (y) ? (x) : (y))
+
 typedef enum ltm_err {
   ltm_err_ok = 0,
   ltm_err_invalid_template,
   ltm_err_failed_alloc,
+  ltm_err_not_found,
+  ltm_err_wrong_block_type,
+  ltm_err_done,
 } ltm_err;
 
 #endif
diff --git a/ltm/include/ltm/template.h b/ltm/include/ltm/template.h
index 445fbf9..ed802f1 100644
--- a/ltm/include/ltm/template.h
+++ b/ltm/include/ltm/template.h
@@ -28,6 +28,12 @@ ltm_err ltm_template_compile(ltm_template **out, const char *template);
 ltm_err ltm_template_compile_n(ltm_template **out, const char *template,
                                size_t len);
 
+/**
+ * Free the template instance. After freeing a template, no instances associated
+ * with it are safe to use.
+ */
+void ltm_template_free(ltm_template *template);
+
 /**
  * Represents a specific instance of a template.
  */
@@ -39,8 +45,15 @@ typedef struct ltm_instance ltm_instance;
 ltm_err ltm_template_instantiate(ltm_instance **out,
                                  const ltm_template *template);
 
+/**
+ * Free the given instance, as well as all nested instances.
+ */
+void ltm_instance_free(ltm_instance *instance);
+
 typedef enum ltm_instance_block_type {
   ltm_instance_block_type_buf = 0,
+  ltm_instance_block_type_buf_owned,
+  ltm_instance_block_type_nested,
 } ltm_instance_block_type;
 
 /**
@@ -57,4 +70,23 @@ ltm_err ltm_instance_block_add_var(ltm_instance *instance, const char *name,
 ltm_err ltm_instance_block_add_nested(ltm_instance **out,
                                       ltm_instance *instance, const char *name);
 
+/**
+ * Calculate the size of the resulting output of this instance.
+ */
+size_t ltm_instance_size(const ltm_instance *instance);
+
+/**
+ * Write at most `len` bytes to the given buffer.
+ *
+ * @param written outputs how many bytes were written to `buf`
+ * @param buf buffer to write to
+ * @param len length of the buffer
+ * @param instance instance to write
+ * @return `ltm_err_ok` if write was successful but there's more to be written,
+ * `ltm_err_done` if everything has been written successfully, or some other
+ * error code
+ */
+ltm_err ltm_instance_write(size_t *written, char *buf, size_t len,
+                           ltm_instance *instance);
+
 #endif
diff --git a/ltm/src/ltm_instance.c b/ltm/src/ltm_instance.c
index 6f9af67..0db85c2 100644
--- a/ltm/src/ltm_instance.c
+++ b/ltm/src/ltm_instance.c
@@ -1,3 +1,7 @@
+#include <stdbool.h>
+#include <string.h>
+
+#include "ltm/common.h"
 #include "ltm/template.h"
 #include "ltm/template_internal.h"
 
@@ -25,6 +29,32 @@ ltm_err ltm_instance_init(ltm_instance **out) {
   return ltm_err_ok;
 }
 
+void ltm_instance_free(ltm_instance *instance) {
+  if (instance->vars != NULL) {
+    free(instance->vars);
+  }
+
+  ltm_instance_block *block = instance->blocks.head;
+
+  while (block != NULL) {
+    switch (block->type) {
+    case ltm_instance_block_type_buf_owned:
+      free(block->data.ptr);
+      break;
+    case ltm_instance_block_type_nested:
+      ltm_instance_free(block->data.ptr);
+      break;
+    case ltm_instance_block_type_buf:;
+    }
+
+    ltm_instance_block *temp = block->next;
+    free(block);
+    block = temp;
+  }
+
+  free(instance);
+}
+
 ltm_err ltm_template_instantiate(ltm_instance **out,
                                  const ltm_template *template) {
   ltm_instance *instance;
@@ -32,15 +62,20 @@ ltm_err ltm_template_instantiate(ltm_instance **out,
 
   instance->template = template;
 
-  ltm_instance_block ***vars =
-      malloc(template->names.len * sizeof(ltm_instance_block **));
+  ltm_instance_block ***vars = NULL;
 
-  if (vars == NULL) {
-    return ltm_err_failed_alloc;
+  if (template->names.len > 0) {
+    vars = malloc(template->names.len * sizeof(ltm_instance_block **));
+
+    if (vars == NULL) {
+      ltm_instance_free(instance);
+
+      return ltm_err_failed_alloc;
+    }
+
+    instance->vars = vars;
   }
 
-  instance->vars = vars;
-
   for (size_t block_index = 0; block_index < template->blocks.len;
        block_index++) {
     ltm_template_block *template_block = &template->blocks.arr[block_index];
@@ -48,7 +83,7 @@ ltm_err ltm_template_instantiate(ltm_instance **out,
     switch (template_block->type) {
     case ltm_template_block_type_literal: {
       ltm_instance_block *block;
-      LTM_RES(ltm_instance_block_init(&block));
+      LTM_RES2(ltm_instance_block_init(&block), ltm_instance_free(instance));
 
       block->type = ltm_instance_block_type_buf;
       block->data.ptr = template_block->data.ptr;
@@ -67,6 +102,7 @@ ltm_err ltm_template_instantiate(ltm_instance **out,
     case ltm_template_block_type_nested:
       *vars = &instance->blocks.tail->next;
       vars++;
+      break;
     }
   }
 
@@ -74,3 +110,151 @@ ltm_err ltm_template_instantiate(ltm_instance **out,
 
   return ltm_err_ok;
 }
+
+ltm_err ltm_instance_block_add_var(ltm_instance *instance, const char *name,
+                                   ltm_instance_block_type type, void *data,
+                                   size_t len) {
+  const ltm_template *template = instance->template;
+
+  ltm_template_block_name *block_name = NULL;
+  size_t i = 0;
+
+  for (i = 0; i < template->names.len; i++) {
+    block_name = &template->names.arr[i];
+
+    if (strncmp(name, block_name->name.s, block_name->name.len) == 0) {
+      break;
+    }
+  }
+
+  if (i == template->names.len) {
+    return ltm_err_not_found;
+  }
+
+  ltm_template_block *template_block = &template->blocks.arr[block_name->index];
+
+  if (template_block->type != ltm_template_block_type_var) {
+    return ltm_err_wrong_block_type;
+  }
+
+  ltm_instance_block *block;
+  LTM_RES(ltm_instance_block_init(&block));
+
+  block->type = type;
+  block->data.ptr = data;
+  block->data.len = len;
+
+  // We insert the block in the linked list and replace its next pointer as the
+  // new attachment point for this variable
+  block->next = *instance->vars[i];
+  *instance->vars[i] = block;
+  instance->vars[i] = &block->next;
+
+  return ltm_err_ok;
+}
+
+ltm_err ltm_instance_block_add_nested(ltm_instance **out,
+                                      ltm_instance *instance,
+                                      const char *name) {
+  const ltm_template *template = instance->template;
+  bool matched = false;
+
+  size_t i = 0;
+  for (i = 0; i < template->names.len && !matched; i++) {
+    ltm_template_block_name *block_name = &template->names.arr[i];
+    matched = strncmp(name, block_name->name.s, block_name->name.len) == 0;
+  }
+
+  if (!matched) {
+    return ltm_err_not_found;
+  }
+
+  ltm_template_block *template_block = &template->blocks.arr[i];
+
+  if (template_block->type != ltm_template_block_type_nested) {
+    return ltm_err_wrong_block_type;
+  }
+
+  ltm_instance_block *block;
+  LTM_RES(ltm_instance_block_init(&block));
+
+  ltm_instance *nested;
+  LTM_RES2(ltm_template_instantiate(&nested, template_block->data.ptr),
+           free(block));
+
+  block->type = ltm_instance_block_type_nested;
+  block->data.ptr = nested;
+
+  // We insert the block in the linked list and replace its next pointer as the
+  // new attachment point for this variable
+  block->next = *instance->vars[i];
+  *instance->vars[i] = block;
+  instance->vars[i] = &block->next;
+
+  *out = nested;
+
+  return ltm_err_ok;
+}
+
+size_t ltm_instance_size(const ltm_instance *instance) {
+  size_t total = 0;
+  ltm_instance_block *block = instance->blocks.head;
+
+  while (block != NULL) {
+    switch (block->type) {
+    case ltm_instance_block_type_buf:
+    case ltm_instance_block_type_buf_owned:
+      total += block->data.len;
+      break;
+    case ltm_instance_block_type_nested:
+      total += ltm_instance_size(block->data.ptr);
+      break;
+    }
+
+    block = block->next;
+  }
+
+  return total;
+}
+
+ltm_err ltm_instance_write(size_t *written, char *buf, size_t len,
+                           ltm_instance *instance) {
+  while ((*written < len) && (instance->blocks.current != NULL)) {
+    ltm_instance_block *current = instance->blocks.current;
+
+    switch (current->type) {
+    case ltm_instance_block_type_buf:
+    case ltm_instance_block_type_buf_owned: {
+      size_t cap =
+          LTM_MIN(current->data.len - instance->written, len - *written);
+      memcpy(&buf[*written], current->data.ptr, cap);
+      *written += cap;
+      instance->written += cap;
+
+      if (instance->written == current->data.len) {
+        instance->blocks.current = current->next;
+        instance->written = 0;
+      }
+    } break;
+    case ltm_instance_block_type_nested: {
+      size_t nested_written = 0;
+      ltm_err res = ltm_instance_write(&nested_written, &buf[*written],
+                                       len - *written, current->data.ptr);
+      *written += nested_written;
+
+      switch (res) {
+      case ltm_err_done:
+        instance->blocks.current = current->next;
+        instance->written = 0;
+        break;
+      case ltm_err_ok:
+        break;
+      default:
+        return res;
+      }
+    } break;
+    }
+  }
+
+  return instance->blocks.current == NULL ? ltm_err_done : ltm_err_ok;
+}
diff --git a/ltm/src/ltm_template.c b/ltm/src/ltm_template.c
index 13ab14a..03dcbd3 100644
--- a/ltm/src/ltm_template.c
+++ b/ltm/src/ltm_template.c
@@ -13,6 +13,30 @@ ltm_err ltm_template_init(ltm_template **out) {
   return ltm_err_ok;
 }
 
+void ltm_template_free(ltm_template *template) {
+  if (template->blocks.len > 0) {
+    for (size_t i = 0; i < template->blocks.len; i++) {
+      ltm_template_block *block = &template->blocks.arr[i];
+
+      switch (block->type) {
+      case ltm_template_block_type_nested:
+        ltm_template_free(block->data.ptr);
+        break;
+      case ltm_template_block_type_var:
+      case ltm_template_block_type_literal:;
+      }
+    }
+
+    free(template->blocks.arr);
+  }
+
+  if (template->names.len > 0) {
+    free(template->names.arr);
+  }
+
+  free(template);
+}
+
 ltm_err ltm_template_name_append(ltm_template *template, const char *s,
                                  size_t len, size_t index) {
   ltm_template_block_name *names =
diff --git a/ltm/src/ltm_template_compile.c b/ltm/src/ltm_template_compile.c
index 7093093..b69aa4b 100644
--- a/ltm/src/ltm_template_compile.c
+++ b/ltm/src/ltm_template_compile.c
@@ -104,20 +104,26 @@ ltm_err ltm_template_compile_n(ltm_template **out, const char *s, size_t len) {
   while (ltm_template_next_placeholder(&ph, s, len)) {
     // Add part before placeholder as literal
     if (!in_nested && (ph.start != s)) {
-      LTM_RES(ltm_template_block_append(
-          template, ltm_template_block_type_literal, (void *)s, ph.start - s));
+      LTM_RES2(ltm_template_block_append(template,
+                                         ltm_template_block_type_literal,
+                                         (void *)s, ph.start - s),
+               ltm_template_free(template));
     }
 
     switch (ph.type) {
     // Invalid placeholders can be detected as early as possible
     case ltm_placeholder_type_invalid:
+      ltm_template_free(template);
+
       return ltm_err_invalid_template;
     case ltm_placeholder_type_var:
       if (!in_nested) {
-        LTM_RES(ltm_template_block_append(template, ltm_template_block_type_var,
-                                          NULL, 0));
-        LTM_RES(ltm_template_name_append(template, ph.name.s, ph.name.len,
-                                         template->blocks.len - 1));
+        LTM_RES2(ltm_template_block_append(
+                     template, ltm_template_block_type_var, NULL, 0),
+                 ltm_template_free(template));
+        LTM_RES2(ltm_template_name_append(template, ph.name.s, ph.name.len,
+                                          template->blocks.len - 1),
+                 ltm_template_free(template));
       }
       break;
     case ltm_placeholder_type_nested_start:
@@ -133,6 +139,8 @@ ltm_err ltm_template_compile_n(ltm_template **out, const char *s, size_t len) {
     case ltm_placeholder_type_nested_end:
       // This means there's more nested end placeholders than nested starts
       if (cur_nested_depth == 0) {
+        ltm_template_free(template);
+
         return ltm_err_invalid_template;
       }
 
@@ -141,17 +149,22 @@ ltm_err ltm_template_compile_n(ltm_template **out, const char *s, size_t len) {
       if (in_nested && (cur_nested_depth == nested_depth)) {
         size_t nested_len = ph.start - nested_start;
         ltm_template *nested_template;
-        LTM_RES(
-            ltm_template_compile_n(&nested_template, nested_start, nested_len));
+        LTM_RES2(
+            ltm_template_compile_n(&nested_template, nested_start, nested_len),
+            ltm_template_free(template));
         LTM_RES(ltm_template_block_append(
             template, ltm_template_block_type_nested, nested_template, 0));
-        LTM_RES(ltm_template_name_append(template, nested_name, nested_name_len,
-                                         template->blocks.len - 1));
+        LTM_RES2(ltm_template_name_append(template, nested_name,
+                                          nested_name_len,
+                                          template->blocks.len - 1),
+                 ltm_template_free(template));
 
         in_nested = false;
       }
       // We encountered a nested end without a start
       else if (!in_nested) {
+        ltm_template_free(template);
+
         return ltm_err_invalid_template;
       }
       break;
@@ -163,13 +176,16 @@ ltm_err ltm_template_compile_n(ltm_template **out, const char *s, size_t len) {
 
   // Unfinished nested
   if (in_nested) {
+    ltm_template_free(template);
+
     return ltm_err_invalid_template;
   }
 
   // Add remaining trailing literal
   if (len > 0) {
-    LTM_RES(ltm_template_block_append(template, ltm_template_block_type_literal,
-                                      (void *)s, len));
+    LTM_RES2(ltm_template_block_append(
+                 template, ltm_template_block_type_literal, (void *)s, len),
+             ltm_template_free(template));
   }
 
   *out = template;
diff --git a/ltm/test/instance.c b/ltm/test/instance.c
new file mode 100644
index 0000000..b2a978b
--- /dev/null
+++ b/ltm/test/instance.c
@@ -0,0 +1,33 @@
+#include "test.h"
+
+#include "ltm/common.h"
+#include "ltm/template.h"
+#include "ltm/template_internal.h"
+
+void test_single_placeholder() {
+  const char *s = "Hello, {{ world }}!";
+
+  ltm_template *template;
+  TEST_ASSERT(ltm_template_compile(&template, s) == ltm_err_ok);
+
+  ltm_instance *instance;
+  TEST_CHECK(ltm_template_instantiate(&instance, template) == ltm_err_ok);
+
+  TEST_CHECK(ltm_instance_block_add_var(instance, "world", ltm_instance_block_type_buf, "World", 5) == ltm_err_ok);
+
+  TEST_CHECK(ltm_instance_size(instance) == 13);
+
+  char buf[13];
+  size_t written = 0;
+  TEST_CHECK(ltm_instance_write(&written, buf, 13, instance) == ltm_err_done);
+  TEST_CHECK(written == 13);
+  TEST_CHECK(strncmp(buf, "Hello, World!", 13) == 0);
+
+  ltm_instance_free(instance);
+  ltm_template_free(template);
+}
+
+TEST_LIST = {
+  { "instance single placeholder", test_single_placeholder },
+  { NULL, NULL }
+};
diff --git a/ltm/test/compile.c b/ltm/test/template_compile.c
similarity index 97%
rename from ltm/test/compile.c
rename to ltm/test/template_compile.c
index c7c7151..127b5fc 100644
--- a/ltm/test/compile.c
+++ b/ltm/test/template_compile.c
@@ -1,6 +1,6 @@
-#include "ltm/common.h"
 #include "test.h"
 
+#include "ltm/common.h"
 #include "ltm/template.h"
 #include "ltm/template_internal.h"
 
@@ -23,6 +23,8 @@ void test_single_placeholder() {
   TEST_CHECK(template->names.arr[0].name.s == s + 10);
   TEST_CHECK(template->names.arr[0].name.len == 5);
   TEST_CHECK(template->names.arr[0].index == 1);
+
+  ltm_template_free(template);
 }
 
 void test_single_placeholder_trailing() {
@@ -48,6 +50,8 @@ void test_single_placeholder_trailing() {
   TEST_CHECK(template->names.arr[0].name.s == s + 10);
   TEST_CHECK(template->names.arr[0].name.len == 5);
   TEST_CHECK(template->names.arr[0].index == 1);
+
+  ltm_template_free(template);
 }
 
 void test_single_nested() {
@@ -83,6 +87,8 @@ void test_single_nested() {
   TEST_CHECK(nested_template->names.arr[0].name.s == s + 33);
   TEST_CHECK(nested_template->names.arr[0].name.len == 3);
   TEST_CHECK(nested_template->names.arr[0].index == 1);
+
+  ltm_template_free(template);
 }
 
 void test_unclosed_placeholder() {