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Press lightly with your pencil to make a faint circle. Sketch the circle so it's as wide as you'd like the whole skull to be. You'll use this outline to make the top portion of the skull. If you're struggling to draw a circle, use a compass or trace a round object that's as large as you want the skull to be. Before you start shading in the skull, take an eraser and remove the horizontal and vertical guidelines that are still visible. Lightly erase the lines for the circle as well. Take care to not erase the actual drawing when you're removing the guidelines.
Make a circle. Erase unnecessary guidelines.