Moons are fun to do but they range from passive eye candy to interact-able entities depending on the implementation.

1. Adding moons with asteroid defs. Purely cosmetic and requires turning on this effect to see them. Allows for more Random placement of Moons.
2. Adding moons as particle effects on the planet mesh. Purely cosmetic and not random as the particle effect is pre-loaded into the planet mesh file. Requires adding more meshes to achieve a feel of more randomness. Can achieve a pretty good variation if all 5 slots are filled per planet entity.
3. Adding moons as an part of the actual planet mesh. Doesn't require particles so it might seem easier. I'd suggest the particle route first.
4. Adding moons as planet items. The goal here is to place an entity that looks like a moon in the planet's gravity well. This approach allows for a random variable amount of moons to be placed. The moons must be a frigate or starbase entity type in order to be placed on uncolonized planets (No Planet Owner). Downside is you may end up with a lot of moons as there's no way to conditionally place planet items. You'll also need to come up with a purpose for them.

IIRC he uses the particle effect approach.

Welcome to the wonderful skillset of reverse engineering. This is a useful skillset to learn.

Good luck.