Making a skin (paint job) for a helmet is quite simple. All you need to do is create a copy of the HEL_default.jpg file, rename it to something like HEL_henri.jpg, and then edit it. Make sure you then select the appropriate helmet skin in-game.
The same for each car you want to skin XFG, etc.
On the other hand, if you wanted to make an entirely new car, so that it's a different shape and has different characteristics, I'm afraid that isn't currently possible.
This forum is run by a few LFS skinners, and you might find it helpful and more in depth. In particular,
this thread (A Beginner's Guide to Skinning) has a good explanation.