Just like any other part. You need a LUA controller, inputmaps, input actions file, the model in DAE, materials, textures and Jbeam. I have a mod for this but it's probably way too complex to understand for a beginner, check any other mod that adds JATO to a car and see how it's done.
Doesn't matter if it's a car or a prop, it's done the same way. You would just have to make a separate version like the Mattress Sport, but instead of wheels and engine you add JATO.
LUA is not a BeamNG specific thing so there probably is one online. But you can just copy the whole LUA file from the bus, it's flexible enough that it will work on anything if you set it up correctly.
