I answered you already ingame, Brian is the one you need to PM. You must give your acc name, Skin ID of the skin you want to change. And wait for brian to add it ingame. Make sure to provide good file and not broken, you can use imgur. I hope this will help you.
Do it takes long time to change the skin after messaging Brian?
I got my squad carshader in less than 24 hours after the donator send it to brian. Brian contacted me ingame and asked for another file because the first one is broken and after a few minutes the shader was added. Brian will just add your skin once he's not busy. Always look /customskin every hour since brian won't pm you if its get added.