Let's get started.
1. Gather the supplies you need
2. Use up the gas
Unless you treated the gas with a a gasoline stabilizer/conditioner, run the mower to use up the remaining gas (use the mower to chop up the leaves on your lawn so you don't have to rake!). Don't leave untreated gas in the mower over the winter, because the gas deteriorates, and the mower might not start in spring.
If there is treated gas in the tank, remove the gas cap, cover the opening with a plastic food storage bag, and screw the gas cap on over the bag. The covering keeps the gas from leaking when you clean under the mower in the next step.
3. Clean the mower body
Let the mower cool after you use up the gas and then disconnect the spark plug wire so the mower can’t start accidentally while you work on it.
Clean the top of the mower by brushing off loose dirt, grass and dust. Then wipe with a damp cloth.
Tilt the mower on its side with the air filter up, so oil won’t leak into the cylinder.
Scrape the underside of the mower to remove grass deposits. A putty knife works well, but if you’re concerned about scratching the finish, use a hard plastic ice scraper. Clean dirt and grass clumps off the wheels too. Then turn the mower upright. Manufactures don’t recommend using a garden hose to clean the mower unless you cover the electrical system, muffler, air filter and carburetor to keep water out.
Remove the air filter. If you replaced it earlier in the season, just gently tap off loose dirt. Replace the filter if excess dirt doesn't come off, or if you’ve not replaced it this season.
Clean under the drive cover. Brush out any debris and scrape the underside of the cover. For instructions on removing the drive cover, see the repair guide and video How to replace a lawn mower drive cable.
Use a hard stream of water from the hose to clean the grass catcher. Let it dry completely.
4. Tighten fasteners and belts
Tighten loose nuts and other fasteners.
On belt-driven mowers, tighten the belts as needed and check for excessive wear or cracks. Replace damaged belts.
5. Store the mower
Put the mower away in a dry place. To keep the outside clean and to protect it from damage-causing moisture, you can cover it with a mower cover, or a waterproof tarp. If storing the mower in an area with poor ventilation, use a light oil to coat the lawn mower—especially moving parts and the cables—to prevent rust.