Sharp lawn mower blades are essential to maintaining a healthy lawn. Blades that aren't sharp shred the grass blades when you mow instead of cutting them off cleanly. This leaves a ragged edge which can allow easy entry for fungus and disease into your lawn. Rotary lawn mowers have a single blade underneath that connects to the engine shaft with a single bolt. You need to remove the blade from the lawn mower in order to sharpen it.
Grasp the spark plug wire and pull it off of the spark plug. This will prevent the engine from starting while you are working underneath the mower.
Lay the rotary lawn mower down on its side and look underneath. You should see a single blade with a bolt in the center of it.
Grasp the blade with a gloved hand and loosen the bolt with an adjustable wrench. Unscrew the bolt completely and remove. Pull the blade off.
Clean the blade by scrubbing it with a wire brush to remove any rust and grass material. Position the blade in a table vice and clamp it down.
Take a metal file or angle grinder and slowly sharpen the blade. Follow the existing bevel on the edge and sharpen until there are no longer any pits or nicks in the blade.
Remove the blade and balance in the middle with your finger. The blade should stay level and not tip to any side. If it does, remove a little more material from the side it tips toward and re-check its balance.
Put the blade back on the lawn mower in the same fashion that it came off. Screw the bolt back into place and tighten down with your adjustable wrench.
Set the lawn mower back upright and push the spark plug wire back on to the spark plug.