Thought I'd just show you the relatively simple process i go through, in this case for my LOST images i posted recently. I guess ideas come fairly well formed to me by now and often a sketchbook scribble won't be far off the end result. Here's a few of my little back pocket sketchbook pages.
Then all I have to do is scan them, re-size them all onto A4 and do a minimal bit of cleaning up, or repairing any bits i drew wrong or that i drew a bit to be patched on later (nearly alway a hand or foot).
I hope that gives you a bit more insight into my general working methods.