The very first and very rough sketch... I don't believe in lineart (it's a good thing but I'm too hasty for it) so I just scribble around and use very thin and very large brushes (thin for outline, wide for the mass and color areas)
Here I determine the general position of the figure, the pose, light direction and some basic color scheme. I usually make a lot of changes in the process but this is the start. Details might be added just as a reminder - it's no use working on the details if you haven't figured out how they would fit in...

..colors and depth.. no details yet.

Blending and adding some details, major changes on the face for a more "evil" look ;)

details and I changed the background again - which is not a very good thing as it's hard to place a figure in a surrounding of a different color. The background light always affects other object in the picture, so I had to change the greenish skintone to a red one.

For the final version I played around with the "warp" and transform feature for a better perspective. And changed the background again.

..and this is the layer overview in a .pst file. I don't use many layers so it's easier to blend it all together.