Well, that's pretty easy.
First, you need to know how to decode rpk.,
Then, you need to know, which scripts are for which parts in-game
Third is writting a proper java. For your first time (and many more "times" 🙂) you can use Shimubitsu 380hp kit's java from 2.2.1. java pack, and replace superIDs in it.
Tutorials should be in tutorials section, if not, look in Google 😉