To record a demo in CS 1.6 or CS:S you need to bring up the console using the tilde (~) key. In source, the developer console must be enabled thru options->keyboard->advanced->enable developer console.
Next, spectate the person you want to demo, open the console and type “record demoname” (without quotes) where demoname is whatever you want the demo to be named, i.e. the player’s name. When you are done recording, open the console again and type “stop” (again without quotes).
To play a demo, again go into console and type: playdemo demoname
That’s it. Happy demoing.