ive made a couple and by no mean claim to be good, BASICALLY what you need to do is record a demo, then make a movie output cfg you can exec once the demo is started viewing, then you type /cl_avidemo 30 - that will give 30 fps, /cl_avidemo 60 = 60 fps. this will create screenshots you combine to an avi with bmptoavi ( a program - google it) from there youhave a huge ass avi likely .5 - 1G / min or so. windows movie maker will allow youto edit it and make a movie. Adobe premire pro is alot better - allows you to import raw AVI's, sequequnces of frames, wav for sound, and mp3 for sound, in multiple tracks.
Q3MME is a really cool q3 mod that actually will record sound with frames and has alot of cool effects. Fool around and see what you can do, sonyvegas is a good one too for editing movies