Here's a video which outlines all of the steps:

How to Remove Door Card Panel - Audi A4 S4 RS4 - B6 B7 2001-2008 - TOTAL TECHNIK - YouTube

It's quite an easy job really. You should buy some of the cirulcar clips shown in the video BEFORE you remove your door cards. Replacing a door card with snapped or missing circular clips leads to the rattling - as experienced by some people here. So be preared, buy them in advance!

You can buy packs of 5 or 10 on eBay pretty cheaply. Allow for 2 breaking per door to be safe!