As I said, the skinning of units and leaderheads is practially the same. You need to change the .dds files of the unit. For example, if you want to change the skin of the warrior, open the warrior directory. There you should find some .dds files. The interesting one is the warrior_128.dds. Changing this file will change the appearance of the unit. The necessary programs to open and change this file are mentioned in the tutorial.