Color. WestWood ones use colors 19 and 20 (remap) for the main body, dark grey for the windows (say, 53-5

Shape. Get a couple pics of what you want to make. Google Helps.
After you've got the thing looking good, now you need normals.
Provided you're using VXLSE III, Tools->Normals->Autonormals. After that, if you see something look bad, go fix it. After that, Tools->Normals->Smooth Normals.
That's pretty much it. If you need to put it ingame, just name it 'car.vxl' and put it in your directory.