First off I recommend backing up these three files in the "Peter_389" folder. 
Custom_paint_01.dds 
Custom_paint_02.dds
Custom_paint_03.dds



In the folder "UV map for custom skins" you will find a .dds image file of the UV for the Peterbilt 389. Simple using this as a template to create your custom skin using whatever paint program you want.

After you are done save the file as a .dds and name it as one of the three names "Custom_paint_01.dds" "Custom_paint_02.dds" or "Custom_paint_03.dds". Place that file in the "Peter_389" folder and overwrite the files. You skin should now appear ingame when you select it ingame.

You can create up to three custom skins.