Using Milkshape is the easiest way, just open it, open a ESF model (.mdl), and then select one of the menus (blah, i dont wanna open Milkshape, so i'll tell u what i know
), and then select one of the skins u want to edit. When u have done that, click "Export BMP" or something like that. Then u just edit it... when u are done, import it.