Safety first! To sharpen your lawnmower blade, first you have to remove it from the lawnmower. Make sure you pull the spark plug wire so the mower does not start accidentally, then remove the blade bolt and nut by using a block of wood to stop the blade from turning, clean up any extra grass, and then use a professional blade grinder, moving the blade back and forth across the grinder. Make sure you balance the blade, or it can cause vibrations later. Replace the blade, and mow away!