hit escape, go to potions, keyboard. click on advanced, then enable developer's console

then, in game, type in console (the ` or ~ key):
record <name> 2500


that will start recording a demo w/ name <name>next, there are two other things you need to do. first, type in sb_status in console as well, and finally (if you want to be able to hear yourself in the demo) type in voice_loopback 1

when the demo's done, just type stop into console