Brennan has written a fascinating, in-depth description of his process for writing Mortal Coil over at his blog, Why Is a Raven Like a Writing Desk?. If you're working on a game, or thinking about it, I heartily recommend taking a look at it. The particulars might vary, but the sort of process, planning and execution that Brennan describes can be the difference between holding a game with your name on it in your hands and simply talking about a game you'll create some day.
If you've already created a game, care to share a little about your process? I'll do my best to get something together about the Burning Empires process in the next few days.