Heh.. hehe...
Get Adobe Photoshop 7.0 (7 is recommended, anyway)
Get WinMPQ
Get BLP<->TGA converter
Use WinMPQ to get the skin file. (*.blp filter with war3.mpq or war3x.mpq) Make sure Storm.dll from your Warcraft III directory is in the WinMPQ directory. (You can just copy it)
Once you extract the file, convert it into a TGA file. (You might have to add .tga at the end of the file when you save it to get it to work).
Open it up in Photoshop. Make your changes and whatnot and save it again (as a TGA).
Convert it back to BLP (again, may need to add .blp at end).
Navigate to your Warcraft III directory and add the folder Textures, and inside of that your file.
Note: Keep the file name the same as the original. Oh and, if it was in Units and not Textures, make sure to navigate accordingly.
Seeing as this was a quick bit, kind of in a nutshell, go find a real tutorial if you need more help. | 
