If you have the console enabled;
type; record <thenamethedemoshouldhave>
And when you're done, just type; stop
If you want to watch the demo, and maybe record a rulebreaker do;
SHIFT + F2 at the same time. A window will pop up, and you can watch the demo you recorded.
NOTE: If you have a slow computer (like mine(or if the demo's long) you'll have to wait a fair amount of minutes to watch the demo. It is not possible to open with Windows Media Player or VLC Media Player.