Simple. Make a greyscale image, and use the template of the object you want to make a bump map for as a guide. Areas that are high should be dark colored or even black, areas that are low should be light colored or even white. If it's in between, it's grey. For example: For a vein popping out on the forehead, take an existing bump map for the figure, and on the forehead lightly airbrush a thin, dark, branching line. Make sure it fades into the rest of the bump map, or it will look like a plateau, not a vein. That's the whole theory. White = cracks, deep, black = bumps, high

Remember, kids! Napalm is Nature's Toothpaste!