Hello. Here are the basic steps:
1. If you were running in salt water, flush the unit with fresh water.
2. Add fuel stabilizer to the fuel tank. This is critical or you will end up with gum in the fuel and be working on the motor in the spring! I suggest doing this first so stabilized fuel gets into the motor. Run the motor for a few minutes to be sure the stabilized fuel gets all the way through (using the flush hose, of course).
3. Run non-toxic antifreeze through the motor cooling system. The easiest way to do this is with a winterizing kit available from most marine stores, which has a large tank with a hose attachment. Put a couple of gallons of antifreeze in the tank and attach the hose to the flush fitting. Alternately you can rig something up or even use a funnel, but it's worth buying the tank kit. Start the motor and run it at idle until you see antifreeze start to come out the back.
4. If the motor is carbureted, when the antifreeze is almost used up or when it has started coming out, spray fogging oil into the air inlet. This might stall the motor - that's fine. If not, turn it off. Don't fog it if its an EFI motor, since that might foul the sensors.
5. Some people like to also remove the plugs and spray more fogging oil into the cylinders - it adds protection.
6. Remove the battery and store it indoors. Check the water level and fill with distilled water if necessary. Don't worry about the charge level, but in the spring put it on a charger a few days before you are ready to get the season started.
7. If it's stored on a trailer, some owners reduce the tire pressure a little, or jack it up onto blocks. This would be a good time to grease the bearings.
8. Do any additional maintenance, cleanup,waxing, etc. you desire, then cover her up and you are good to go.
Marine Mechanic
USCG Licensed Captain, 20+ Years Boating, Fishing, Maintenance and Repair Experience