ffdn.org/sites/all/modules/footnotes/footnotes_wysiwyg/footnotes_wysiwyg.module

<?php

/**
 * @file
 * WYSIWYG support for the footnotes module.
 * Currently this supports TinyMCE and CKEditor via the WYSIWYG module and
 * CKEditor via the CKEditor module.
 */

/**
 * Implementation of hook_wysiwyg_plugin().
 */
function footnotes_wysiwyg_wysiwyg_plugin($editor, $version) {
  $plugin = array(
    'footnotes' => array(
      'buttons' => array('footnotes' => t('Add Footnote')),
      'basePath' => base_path(),
      // This is required to auto-load the plugin.
      'load' => TRUE,
    ),
  );
  switch ($editor) {
    case 'tinymce':
      $plugin['footnotes']['filename'] = 'editor_plugin.js';
      $plugin['footnotes']['path'] = drupal_get_path('module', 'footnotes_wysiwyg') . '/tinymce_plugin/';
      $plugin['footnotes']['extended_valid_elements'] = array('fn');
      return $plugin;
    case 'ckeditor':
      // For CKEditor, path is the directory and the .js file must be called 'plugin.js'.
      $plugin['footnotes']['path'] = drupal_get_path('module', 'footnotes_wysiwyg') . '/ckeditor_plugin/';
      return $plugin;
  }
}

/**
 * Implements hook_ckeditor_plugin().
 *
 * Register the CKEditor plugin for the CKEditor module.
 */
function footnotes_wysiwyg_ckeditor_plugin() {
  return array(
    'footnotes' => array(
      'name' => 'footnotes',
      'desc' => t('Add Footnotes'),
      'path' => drupal_get_path('module', 'footnotes_wysiwyg') . '/ckeditor_plugin/',
      'buttons' => array(
        'footnotes' => array(
          'icon' => 'images/footnotes.png',
          'label' => 'Footnotes',
        )
      ),
    )
  );
}