Here are the programs and templates you need to skin and 2 tutorial videos:
templates
truck: https://ets2.lt/en/templates-for-all-trucks/#more-25214
trailer: https://ets2.lt/en/trailers-template/
programs:
something like photoshop (I use this program: http://www.gimp.org/downloads/)
program to make your skins to mods: http://www.ets2studio.com/p/download.html
videos
truck: https://www.youtube.com/watch?v=Vij8rrzfprI
trailer: https://www.youtube.com/watch?v=k1sOXXm5v4g
Hope you can make your own skins now 😉