That's the most generic question. There are different kind of mods: Vehicles, Levels, Objects, Addon Parts, Skins, etc. etc Which one are you interested in? You can find most guides here: https://www.beamng.com/forums/content-creation.29/ And here: Portal
But for my mod, do I build the lights too? --- Post updated --- i am having trouble with Resource Files thing... to save my mod
You still haven't said what kind of mods you are even trying to do. Making a vehicle is not the same as making a level, or an addon part :\