First of all you need Warcraft3 Viewer to extract a model file. When you extact it, you'll need W3 File Converter to change .mdx to .mdl Then you need Vertex Modify to do changes on the model, you can also add a existing skin in wc3 with the help ıf this program (to do that you have to first extract skin). If you want to use your own skin use photoshop or paintshop pro. Read some tutorials and you'll understand all.