A riding lawn mower's blades should be sharpened at least twice each season. The process can be done quickly when you know how. The process is the same for a riding mower blade as it is for a push mower, with the exception of a riding mower needing the mowing deck removed first. However, those instructions are usually mounted on the mower deck to guide you through the process.
Remove the mowing deck from your riding lawn mower using either the removal guide stamped on the deck itself or the owner's manual. After the deck is removed, flip the deck upside down so the blades can be removed.
Mark the bottom of blade with a grease marker or shot of spray paint. Doing this ensures you will re-install the blade correctly. A blade that is installed backward will not cut grass. One way to know if the blade is reversed is to see what direction the cutting bend is. A correctly installed blade has the angle pointed down.
Use a socket wrench to remove the bolt that secures the blade to the mower deck. You may need to insert a block of wood between the blade and the inside edge of the mower deck to prevent the blade from moving while removing the center bolt.
Remove rust from (and clean) the mower blade by rubbing it with steel wool.
Clamp the center of the mower blade in a bench vise. If you do not have a bench vise, you can secure the blade to a work table with a C-clamp.
Use a common multipurpose file to grind the edge of the cutting surface on the mower blade. File at the same angle that was milled on the blade. Remove as little metal as possible while filing. If you do not have a file, you can sharpen the blade with a grinding wheel. Special care should be taken when using a grinder not to heat the blade, which could cause the blade to bend.
Unclamp the blade and turn it over to file the opposite side. File the blade in the same manner as before.
Unclamp the blade after filing and check the balance of the blade by inserting a screwdriver through hole in the blade where the mounting bolt attaches. A properly filed blade will balance on the screwdriver blade.
If one side of the blade dips, balance by removing small amounts of metal from the end edge of the blade with a file. Re-inspect the balance by repeating Step 8.
Reinstall the blade back on the mower deck using the socket wrench and block of wood, as needed. Then reinstall the mower deck to your riding lawn mower.