Mercury

Mercury is the smallest planet in the Solar System and the closest to the Sun.

It's the simplest one out of the bunch in RSS, terrain wise, so if you want to learn how to make texture based terrain, take a look at it in planet studio. You can save it under a different name and use it as a template too.

Just note that JNO the game has extensive features for procedural generation of terrain, that's the way stock does it, not with textures. You shouldn't view this – cubemap textures – as the way to make custom planet surfaces, but rather as a last resort if you need really specific terrain.

Convert equirectangular to cube textures with https://jaxry.github.io/panorama-to-cubemap/

Imported textures are stored within the UserData/SupportFiles folder in the AppData/LocalLow game directory. I usually first import them (+ sign after clicking on one of the textures in the texturedata pane in the terrain gen pane), and then go to that folder to F2, copy, and paste at the beginning of the xml according to the id (px matches up with px, nz with nz...)