Lawn mower blades require proper sharpening to operate effectively. Lawn mower blades that are dull will rip and shred at your grass when you mow. The ripped and shredded grass allows disease to enter which will harm or possibly kill your grass. Sharpen lawn mower blades at least once per year---more often if you consistently run into sticks and other debris.
Move the lawn mower to a smooth and well-lit surface to remove the blade.
Disconnect the spark plug wire by pulling it off of the spark plug, and lay the lawn mower down on its side.
Locate the bolt that holds the lawn mower blade to the engine shaft. It is in the center of the lawn mower blade. Use an adjustable wrench to remove the bolt. Remember the order the washers were in so that you can properly reinstall the blade once it's sharpened.
Remove the lawn mower blade, and take it over to your table vice. Place it in the vice, and tighten the vice down. Use a wire brush to clean the entire blade so you can see what you are working with.
Take a metal file or angle grinder, and sharpen the lawn mower blade. Follow the existing edge of the blade and sharpen that edge, but don't alter it in any way.
Hold the lawn mower blade in the middle to check its balance. It should not tip in any direction. If it does, remove more material on the side that it tips toward.
Put the lawn mower blade back on the lawn mower, put the bolt and washers back on and tighten down.
Set the lawn mower back upright, and reconnect the spark plug wire by pushing it onto the spark plug.