Ok. This should be where you stick the files.
C:\Program Files\Steam\steamapps\Player\counter-strike source\cstrike
(Please be aware that "Player"= Your steam account name.)
Once you download the files there, you will go into Counter Strike. Once in, you access the Developer's Console and type in playdemo <name>.
If you feel REALLY lucky, you can press f2 and edit the demo. But I have yet to use this option, so good luck on it.