To do skins, you get a skinning program like the ones I listed above, but before you do that you need a tool which can export skins (the .blp) from your wc3 mpq and that could be WC3 Viewer.
It takes some skill and time but you work then convert the extracted .blp to a compitable image format for the program to work on. Skinning means to edit or redesign a existing texture. (I can skin so ask questions if you need)
Skins are basically " clothes " or you can say " the wrapping " for a model.
Remember, skins must be .blp to work.
To get skins working, import them via Import Manager (must have WC3 TFT) or WinMPQ and the path the page for the skin has told you, then the skin will automatically apply itself to the existing model.