How do I create a new part and add it to a car?

Hello, all. I have some experience with editing the meshes of a car (fenders, bumpers, etc..) and doing engine swaps, and stuff like that, but I've tried in the past to create a new part in Blender and add it, and either the materials are missing, or the Jbeam is wrong and I have to add it from another part, or something else. Are there any tutorials out there for creating a part from scratch and implementing it on a car? Thanks.