1. Remove blade from mower. Disconnect the spark plug or unplug the mower. Then turn the mower over and remove the nut that attaches the blade to the motor. Then remove the blade.
2. File the blade. Stabilize the blade in a vise and use a medium file to smooth out imperfections along the blade edge (at the ends of mulching blades). Don't try to hone a sharp edge.
3. Check for balance. After you smooth the cutting edge on both halves of the blade, check to see if the blade is balanced. If not, remove more metal from one of the halves until the blade does balance.
4. Reassemble mower. Be sure to tighten the nut securely.