Here is a step by step. I don't always do all these steps, but this is fairly representative of what I do most of the time.
- A quick sketch, usually done in Photoshop with a Wacom tablet and a hard variable brush. I toggle between black to draw and white to "erase" in order to work quickly and energetically. I'd hate to be stiff at this point.
- I print out the rough sketch and do a clean pencil drawing on vellum to work out all the essential details (I don't always do this, but when the art director is being finicky and I want to make sure to get the details right I'll do this to get an approved sketch)
- I scan the drawing in, set it on a layer with "multiply" and block in values on a layer underneath. I flip the drawing often to make sure it looks right, flipping reveals flaws. I usually like to colorize the line drawing to a warm sepia color. I don't paint directly on the drawing layer because I want to be able to separate the lines to keep the painting clean of drawing marks. (yet I can still keep the drawing in places where I want to hold the line)
- Block in local color very transparently, sometimes this is just colorizing the grayscale block in.
- Block in the lights opaquely with a rough textured brush, this is to get the impression of more detail and texture in the light (in nature light reveals detail while shadow obscures detail, this process simulates that effect)
- Render the form in more detail. I am no longer colorizing the drawing, but actually introducing colored brush strokes to mix into the painting. This helps avoid the colorized drawing look, and makes it look a little more like a painting.
- Highlights and sparkles. Those finishing touches that make an illustration look, well, finished.
