Each mower is a little different, and you should consult your owner’s manual for the specifics of your machine, but these basics apply to every mower.
Before you work on a lawn mower, always remove the wire going to the spark plug so that there is no way that you can accidentally start the mower.
Clean the underside of the mower thoroughly, scraping off dried-up grass clippings with a putty knife. Unplug any air vents and channels.
Unscrew the spark plug with a deep-socket wrench that fits the hexagonal nut. Install a new plug of the exact same type, being careful not to cross-thread it (setting it in askew, so the threads get damaged as you turn it). Tighten gently.
Unscrew the engine cover and vacuum up any dirt and grass clippings.
Unscrew the oil dipstick and carefully tip the mower onto its side, allowing the dirty oil to drain into a pan, such as an old pie tin. (Some mowers have an oil drain plug that can be removed with a wrench, so the oil can be drained without tilting the mower.) Check the owner’s manual for the correct oil type, and pour in new oil until the dipstick reads “full.” Clean up any spilled oil with a rag.
Unscrew the air-filter cover, remove the filter, and discard it. Clean the filter area thoroughly with a rag and an old toothbrush. New paper air filters are simply reinserted with the pleated side facing out. Foam filters should be soaked in clean, new motor oil; squeeze out the extra oil onto a rag, then insert the foam filter into the proper position. Replace the cover.
Before storing a mower for the winter, run it under supervision and in a safe location until the gas tank is empty. Disconnect the gas line and, if possible, lift off or unscrew the gas tank and clean it by putting half a cup of fresh gas inside and swishing it vigorously. Do not store gasoline in any container over the winter; always use fresh gas in the spring.