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Centering the clay means that its outer edges spin perfectly smooth with no bumps or wobbles. There are different ways of doing this, and there are two methods described here. Use whichever one will work best. The beginning is the same for both methods. Make sure your body is centered over the wheel and legs are planted firmly on the ground.
Make sure that your arms are firmly in position and pressed into your thighs. Start by cupping the clay in both hands with your thumbs over the top. Make sure that your hands and the clay are always wet. Pull your wrists back slightly while pressing down and holding them rigid. Start compressing and smoothing the clay with your palms. Only do slight movements, a little pressure goes a long way.