I used to confused about mods and stuff when I first started playing source games. There is a method to installing any skin. There's 3 types of skins, which I call Simple ones, Packaged ones and Selection ones.
The Simple ones when you open up the .zip you'll instantly find the Models, Materials, Scripts etc. and you can simply extract that to your TF2 folder.
The Packaged ones are the same as the Simple ones except that when you open them up at first they might have a folder or folder that has multiple names on it. You have to navigate through them until you find the Models, Materials, etc. and then copy over those folders to your TF2 folder.
The Selection ones are skins which have different accessories, these are usually found in CS:S skins. To deal with them, you will find folders which tell you which enhancement they give and there will normally be a folder which you have to extract no matter what. Select what features you want and treat each of them like Simple ones.