Well, i dont know how to make a mod buy i know how to edit skins,paint jobs,clothes,buildings etc.....
To edit the skin or colour download TXDWORKSHOP then open a txd file, then go on properties and decompress. Then export the picture you are going to mod and save it, then open 2 paints and get a picture you want in the 1st 1 then in the 2nd 1 get the file u want 2 mod. With the paint with the picture u want to add in select what part u want and paste it. Then save it and open TXD WORKSHOP and import the file u edited 2 the same 1 and VIOLA JUST SAVE AS!
(I hope i made it clear, for more clear instructions go on utube)
I have attached TXD WORKSHOP to save you time
