Simply extract everything to your C:/Program Files/Valve/SteamApps/*USERNAME*/counter strike source/cstrike folder. That should be right, correct me if I'm wrong.
Most of them use RAR, that I've found. You need WinRAR for that.
Cryptex - that only works if they only compressed the materials/models/etc. folders. If they tossed it into a "my gun models" folder, then just extracting it into cstrike won't work.
Well, another method I use is I made a folder in my My Documents folder called "CSS Skins" Extracted everything there (label folders for each type first). After that simple copy all the stuff you need, and post it in your cstrike folder.