Hello, I would like to update my repository of planets with the new terrain system. However, I don't know how to actually make the new terrains. Can someone help me with this?
Making new handmade terrain from scratch and bringing it into the game isn't quite possible yet. I'm working on it. You can generate new procedural terrains with the Terrain Generator. LINK
You can't use custom terrains in MP. There is currently no way to guarantee that all players are using the same terrain.