Either you record straight to Fraps, or you first record a demo by typing "record <demoname>" in console
Then do "playdemo <demoname>" and recording whilst you are playing it back. Pretty complicated but it's easdy once you get the hang of it.
Fraps is quicker and easier, but the quality suffers. And the "good" version is not free.
Acid's old guide sums up pretty much everything you need to do it the "best" way. http://www.themovievault.net/forums/topic/2060
imo his autoexec for vids is lacking, but thats more personal preference and easily added to.