you gotta download a couple of tools, you need a paint program,
first of all, you download wc3 viewer in the tool section
, open a model, convert it to tga using wc3 viewer
open the model with photoshop, paint it up however you like it
save it as a tga file
open wc3 viewer, convert the new skin to bmp
, I explained this really quick and it isnt very detailed, but if you are computer smart you should get it real easy
if you got it, no problem, if you didnt, pm me,