Been years since I started the process off, and I'm sure it's been refined. It's a Frankenstein's monster of Fractal Terrains, Photoshop, Illustrator, and InDesign. Trying to create/replicate maps or space images of the planets (we call them "beauty shots") is friggen' test of valor and patience. Completely new planets can still be difficult to match all the details necessary in the planetary writeup.
I'll ask David Kerber to chime in on the current process.