As soon as it says "FIGHT!", Push the "F5" key to start recording a demo. It will save it as "demo01" in the demos folder in your base Q3 directory.
[f5] Start Recording Demo
[f6] Stop Recording Demo
[f7] Que Prev
[f8] Que Next(ie demo02, demo03 etc)<---this is important because it will record over one of the same name
To view your demos go to the main menu and choose "demos" your demos should show up in a list.