Here are the different steps of my process when working on an illustration:
First I start by doing thumbnails in order to generate as many different ideas as possible. I sometimes do them in my sketchbook, other times directly on the computer:
I then proceed to drawing the final sketch of my drawing. This will define exactly where everything goes and looks in my final drawing, so it is important for it to be very accurate. It will often not be visible in my final drawing, in which cases it doesn't need to be very clean. When I'm planning to let it through, I clean it up more.
Then I work on the color scheme of the piece. Once the sketch is done, I mostly work digitally only. I usually try many different color schemes at this point, in the same way as I draw many thumbnails. This is what the painting looks like after the color scheme has been chosen:
Then I just render, render, render for a very long time.