First, make the chin of the skull by drawing out a slight sharp curve facing downwards. Then, make the forehead and the upper part of the skull by drawing huge slight curves. Draw a circle on the spot where the eye socket should be. Draw a curve at the side of the circle to give it an illusion of popping out. Draw the cheekbone lines as well as the jaw lines to give the skull more definition. Draw several rectangles standing vertically to create the skull's teeth. Finish the head with a slight curve. Draw intersecting zigzag lines to illustrate cracks on the skull. When done, color your artwork as you desire to give it a more realistic look.