Check out the addons page for more information!
To install the skins: you must go to C:\program files\steam\steamapps\(your steam name)\team fortress 2\
the .zip file should already have the needed folder. Just extract it to a folder for safe keeping, and put the folder named "tf" (the first folder you see, hopefully, or else you're doing something wrong.) into your Team Fortress 2 folder. And that should be everything you need!