Unlike new-generation car engines with multiple cylinders and computer controlled electronics seemingly requiring a master's degree to tune them up, lawn mower engines are built fairly simply and need only a bit of common knowledge to do a standard tune-up. Regular tools that almost any do-it-yourself person will own are all that are needed, along with parts that are easily available at any hardware and most department stores.
Change the oil by running the engine for about 1 minute to loosen up dirt particles and make the oil flow easier. Remove the oil plug from beneath the mower deck with your adjustable wrench and drain the used oil into an old container. Replace the oil plug, fill the engine with your quart of oil, check the dipstick and add more if needed.
Pull the rubber boot off the spark plug, fasten the spark plug wrench to it, and remove it in a counter-clockwise direction. Take your new spark plug and place it into the plug hole, hand thread it until tight, firm it up with your spark plug wrench turning in a clockwise direction, and reattach the boot.
Unscrew the air cleaner cover, and most will have a plastic wing nut holding it in. Remove the foam filter element from inside and put it into the soapy bucket of water. Dunk and squeeze several times until the filter is clean and allow to dry before putting it back in.
Locate the fuel filter by tracing the fuel line from the gas tank. It will look like a little plastic barrel. Clamp off the side that is attached to the fuel tank, using either a regular hand clamp or a locking pliers, then unscrew the hose clamps on either side of the filter with your flat blade screwdriver. Pull out the old filter and replace it with the new one, screw on the hose clamps to either side, and remove the hand clamp or locking pliers from the fuel hose.
Clean out any debris from the housing that surrounds the engine. This will allow efficient engine cooling and will extend engine life.