Hello icencrime,

Here are some guidelines how to do it:
Edit the index.php in the root and add something like:

  case 'yourpage':
     include('includes/page.inc.php');
     break;

after:

  case 'disabled':
     include('includes/disabled.inc.php');
     break; 

You will have to create page.inc.php (and call it however you want). Next, you need to edit a link to your page in main.tpl in /themes/[your_theme], something like this:

<a href="index.php?mode=yourpage" title="yourpage">Link to my custom page!</a></li>

To make this complete I created a subtemplate page.inc.tpl for presentation which is called via smarty from page.inc.php

Hope this is not too much over your head. :)