Different body option

I have been making a mod recently and have a saloon model of it that I want to be able to select in the parts selector. I already made a thread like this a while ago but nobody replied and I haven't found any tutorials on how to do it, so can somebody please tell me how I would do this?

Thanks

 

look at how you can select frames for the D-series. It's exactly the same.

 

I tried looking at things like that and did it for my mod, but nothing worked