record a demo
open console and type 'record [put a name for your demo here]'
when you want to stop recording, type stop in console
to play the demo file, press shfit + f2 or type 'demoui' in console to open the demo UI. Click load, and look for your recording, which will be [name you entered].dem. Open the file, and boom. It'll be like spectating a server, so just go to your cam and watch yourself in 3rd person. Shift + f2 for controls like slowing down playback, pausing, or going to a specific tick in the demo.
Hope this helps