As the old saying goes "I could tell you but then I'd have to kill you". Seriously though there isn’t much mystery to the process involved. I’m a full on Photoshop user. I do not stray from my golden god Photoshop. Photoshop praise be.
Basic geometry, texture creation and general layout are all conceived in Photoshop. On the very rare occasion I may dip into FormZ for a 3D render of a surface or insane perspective. The other vital tool of the trade is a Wacom tablet...praise be the Wacom tablet. This is the connection I needed years ago to bring my illustrative talents to the computer screen. It's a pen, an airbrush, paintbrush, pencil, knife, ruler, masking tool...its everything! I feel incredibly lucky to live in a day and age where we have tools like this at our fingertips. And finally, you can’t have enough reference material. Research, research, research!
Seriously though what I rely on more than any of these tools is my history in Graphic Arts and Design. If you can paint or illustrate with some sense of photo-realism you'll do well in this industry. I have a basic work process that relies heavily on many learned disciplines over the years. Oh ya, there’s also the holy grail of filters in Photoshop..."The talent filter". This thing does everything! Just kidding
Sorry for being so vague but if I gave a step-by-step accounting of process I might be out of a job. If you’ve got some design skills you can do this stuff without me saying a word