Well, first you need the right video editor. If you're open to spending some money, do so. Most editors, however, don't accept dm_68 (the file type for demos), so you need to record video from your demos (/video in the console, /stopvideo to stop). You can find this in a folder that is created in your q3ut4 folder once you record a video. These are primarily .avi, so you may need a converter depending on the editor you have.
As for technical, I made my Zombie TS advertisement on iMovie, with my Mac. But there's the setup for ya