Hello JackieG,
I won't get into the details of or skinning or using gimp as it has be covered many, many times here in the forums. What you need to do is use cmp to locate the item you want to skin and clone it then using the new cloned item "open for edit in explorer" this will open a Windows folder containing several files: a mesh file (.im or pm), config.txt, one or more *.texture.txt files and one or more image files (.tga, .bmp, .jpg). Some items are simple and will have just one .tga or .bmp file to edit others will have multiple. Examine these image files to determine how each applies to the model and then open them with gimp and modify them as you see fit.
search the forum for greater details. You will find everything needed to get you to completion.
Bricey