Example setup to make an entry for Outer Wilds Mod Jam 5.
The most important takeaways are:
#1 Have a center body with "centerOfSolarSystem": true set. All other bodies in your mod must be positioned relative to this. Doesn't have to be a star, can even just be an empty point in space, just has to be there!
#2 Have a platform with
"extras": {
"isPlatform": true
},which will be the starting point for your mod within the central station Nomai vessel!