Drupal 7 Themeing Notes

<?php
/**
 * Implements hook_theme()
 * @return array
 */
function mymodule_theme(){
  $items = array();
  
  $items['mymodule_custom_template'] = array(
    'path' =>  drupal_get_path('module', 'mymodule'),
    'template' => 'mymodule_custom_template',
    'variables' => array(
      'account' => NULL,
      'name' => '',
      'mail' => '',
    ),
  );
  return $items;
}
/**
 * Example usage within an arbitrary function
 */
function _mymodule_example_usage(){
  global $user;
  
  print theme( 'mymodule_custom_template', array( 
    'account' => $user,
    'name' => format_username( $user ),
    'mail' => $user->mail,
    );
}
/**
 * Implements hook_preprocess_HOOK()
 * In this case HOOK is "mymodule_custom_template". The name of our custom template's key in the above array.
 */
function mymodule_preprocess_mymodule_custom_template( &$variables ){
  // link the name to the user's page
  $variables['name'] = l( $variables['name'], "user/{$account->uid}");
}
/**
 * Implements hook_preprocess_HOOK()
 *
 * Render array examples attached to the main page content.
 *
 * @param $variables
 */
function mymodule_preprocess_page( &$variables )
{
    // default render array #type is markup which accepts any HTML in
    // the #markup property
    $variables['page']['content']['my_render_array1'] = array(
        '#markup' => '<p>Arbitrary HTML goes here</p>',
    );
    // example render array type 'html_tag'
    $variables['page']['content']['my_render_array2'] = array(
        '#type' => 'html_tag',
        '#tag' => 'h2',
        '#value' => 'Find Me',
    );
    // render array that uses a theme hook
    // variables are passed to the function as #-prefixed properties
    global $user;
    $variables['page']['content']['my_render_array3'] = array(
        '#theme' => 'mymodule_custom_template',
        '#account' => $user,
        '#name' => format_username( $user ),
        '#mail' => $user->mail,
    );
}

<?php
/**
 * Available variables
 * 
 * $account object - drupal user account
 * $name string - the account's formatted and linked name 
 * $mail string - the account's email address
 */
?><p><strong>Name: <?php print $name; ?></p>
<p><strong>Email: <?php print $mail; ?></p>