Timedemo/Benchmark - How to create one?

For games that don't have built-in performance demos, how do I create my own? I know how to create a demo in the games, and I have FRAPS, but I don't understand how the two are connected.......or is there some other way??

The timedemo that I downloaded for CoD2 is quite simple, it runs through a demo and calculates how long it takes to complete/render though all those frames.....does the simple math and there ya go!

Is there any way I can do this for other games that don't have timedemos to download? I want to make my own but can't quite figure it out.

Any help would be most appreciated!