Check This!, here is all you need to learn how to skin! Sinning is changing the texture of a Wc3 unit.
How to skin?:
Tools you need: - Wc3 Image extractor. Find it in Wc3sear.ch, in the tools category, 4th page if you are showing 15 elements per page. (Sorry for not making a direct url to it, but since I didn't made even 15 posts, I'm unabled to do it)
- Adobe Photoshop or a painting program that can open TGA
How to start:
Well, the Image extractor can open Mpq files to extract its images and change them, then save them in .blp format, which is the extension that blizzard uses for his textures.
Open a .mpq file.(War3.mpq, War3x.mpq(Expansion) ) They are in the Warcraft II folder.
Find out the texture that the model you want to change uses, you can use search, but sometimes Bliizard doesn't use the name you think they use for the texture, some of the textures are in the Textures/ part and others in Units/ part. (For a Demon Hunter: "Textures/HeroDemonHunter.blp", for Archimonde "Units/Demon/Warlock/Warlock.blp(I think))
Then Save it as a TGA. (You can save it as a jpg, but it is better a TGA I think)
Open it in a painting program. Change what you want (Alpha Channel is for team Color and Transparency, you should be careful with this one) Save the image.
Then open the image with the Image extractor, save it as .blp. Go to World Edit, import the .blp in your map with the same path that the original texture had.
Save the map, select the unit you might have changed, and ready!