Download a MPQ extractor (Image Extractor II or WC3 Viewer)
You can download them on this site in the tools section
Open them up if your using Image Extractor II:
1. Browse through the category view to find your skin
2. Once the .blp skin you want has been found, extract it by right clicking and save it as a .tga save the skin somewhere
Open them up if your using WC3 Viewer:
1. The list should quickly come on once you open WC3 Viewer.
2. Browse through the list by its categories to find the skin you want
3. Once its found, right click it and " extract and convert .blp " into a .tga (32bits or .24bits doesn't matter for now)
4. Save the skin somewhere
Your image program (Paint Shop Pro) supports .tga and its the format where the skins were extracted so just open the skins up and start skinning