Well... Paint.net is the software you create the "picture" or texture and notepad++ is used for the files necessary
going off of what what he was saying, some people would rather have lore branding (gavril/bruckell) as it would fit in the lore and not break the immersion, personally I like lore brands more as well but again it is completely up to you if you decide to do it or not.
Paint.net. It's free. You can use .PNG with Beam, but .DDS is preferred because it compresses better "saves spaced when in a .zip" If you use .PNG, BeamNG converts it to .DDS anyway as a temp file, so .DDS can save load time. When you "save as" in paint.net, you can choose to save it as .DDS.
look, all you do is put it in the mod folder go to the mod page in the game go to your mod manager and unpack it. it will show up in the unpacked file in the mod folder, from there all you do is make the skin, edit the "Skin Exmp" to what the skin name is and you're good.