I was asked my process for these. It's pretty simple in terms of steps.
I treat these just like I would when painting in oil. Tone the canvas, do the drawing/design out the
shapes. Wash in the shadow with thin paint ( Key the value by adding the darkest dark and light), then paint opaquely up to the light :)
I sometimes just block in shapes ( I love this way of painting ) but it takes me longer. Usually when I paint
this way I have a game plan already worked out in my head