Do you already have the skin? If so, it depends on what software you're using...
If you're asking how to actually create your skin, well, that too depends on the software, but a good place to start would be by taking a look at other skins and picking apart the code/images to see how they work. You can get certain tips on skinning, but we need to know what software you're using first...and you won't find a tutorial that will tell you everything you need to know about how to create your own skin