[H0ok3r w A P3n1s™]

Teach me how and I promise I'll send the video.

When you're in-game, open up console (~) and type:

[INDENT]record DEMONAME[/INDENT]

...where DEMONAME is whatever you want the file to be called (if you're going to report someone, it's easiest to just use the players name), then press Enter. It will start recording. When you are done recording, open up the console again and type:

[INDENT]stop[/INDENT]

The demo will stop. You can then send us the .dem file.