Having good perspective reference is invaluable. I made a simple mockup of my space habitat in Google Sketchup & took a screenshot to use in PS. It's a fiddly program...but ultimately useful for this!

After taking the screenshot, I deleted everything except the lines in Photoshop, and then colored them white. I drew my rough color sketch underneath so I could figure out what goes where!

Once the rough color sketch was in place, I started working out the colors and details more. (using adjustments/adjustment layers to fiddle with color)

Once I got the color figured, I added in even more rough details. After this, it's just a matter of polishing up the lines and shapes and adding some bits of texture on top!