The process of creating a BeamNG car is quite simple, but often repetitive, tedious, very very finicky and always long winded. A time lapse video of car creation from start to finish would likely run for many hours and involve more than its fair share of swearing. Dissect and tinker with Gabester's jbeams to get an idea of how these cars work. With jbeam, the best way to learn is to experiment. Start off with small static objects and gradually increase the complexity of your projects until you know you're ready to jbeam a whole car. As the saying goes, walk before you run. I hope you already know how to model and texture; you need those skills too.
And then there is the shady way of converting a model and putting it onto a n/b, but people seem to not like the products.