How exactly do I create a variation of a .tpl file? I am new to Drupal and am working on a partially complete site. There's a .tpl file I wish to duplicate and alter and have only be used with a certain Panel Page type ('Product Page').

I've read Drupal's documentation about template suggestions, but am not sure what to actually do.

I've placed the following in my template.php file:

function mytheme_preprocess_node(&amp;$variables) {
  if ($variables['???']) {
    $variables['theme_hook_suggestions'] = 'new_tpl_file';
  }
}


But I've got no idea what to put in $variables[''] so that the suggestion appears on my pages. I know my Panels page is called 'Product Pages' but that's it. Is there a machine name for that I can use as a variable? Would it work if it was something like:

      if ($variables['product_pages']) {


Does anyone know of a tutorial or guide out there? Im pretty lost here. Any help is appreciated!
If you are trying to override panel page template then I am afraid you are using the wrong hook, hook_preprocess_node works for node pages only.

However panel page goes through hook_preprocess_page, so you can add custom template suggestion in following way.

/**
 * Implements template_preprocess_page().
 */
 function MODULEORTHEMENAME_preprocess_page(&amp;$variables) {
    if (module_exists('page_manager') &amp;&amp; count(page_manager_get_current_page())) {
        $variables['theme_hook_suggestions'][] = 'custom__panel';
    }
 }


Then create custom-panel.tpl.php

Reference: 
page.tpl.php override for Panels Pages?