Creating a template file for a form is not different from creating a template file that is not for a form.

Define a theme function, using as theme function the name of the form builder callback. The code should be similar to the following:

/**
 * Implementation of hook_theme().
 */

 function mymodule_theme() {
   return array(
     'mymodule_form' => array(
       'template' => 'mymodule-form',
       'file' => 'mymodule.admin.inc',
       'arguments' => array('form' => NULL),
     ),
   );
 }

If the form contains the value $form['field_1'], its value will be available in the template file as $field_1. The template file will be also able to use any values passed from template_preprocess_mymodule_form().

I have actually never needed to use a template file for a form.
As far as I can see, Drupal core code uses theme functions, when a form, or part of a form needs to be rendered in a particular way; a theme function that calls drupal_render() is normally enough for any purposes.

I have actually never needed to use a template file for a form.
As far as I can see, Drupal core code uses theme functions, when a form, or part of a form needs to be rendered in a particular way; a theme function that calls drupal_render() is normally enough for any purposes.

To reply to the question, creating a template file for a form is not different from creating a template file that is not for a form.

Define a theme function, using as theme function the name of the form builder callback. The code should be similar to the following:

/**
 * Implementation of hook_theme().
 */

 function mymodule_theme() {
   return array(
     'mymodule_form' => array(
       'template' => 'mymodule-form',
       'file' => 'mymodule.admin.inc',
       'arguments' => array('form' => NULL),
     ),
   );
 }

If the form contains the value $form['field_1'], its value will be available in the template file as $field_1. The template file will be also able to use any values passed from template_preprocess_mymodule_form().