Hi,
I work at a mastercraft dealer in Montana and have probably winterized over 100 of these boats in the past few months.
1. Change oil and filter
2. Change fuel filter
3. Pull plugs from exhaust manifolds, engine block and oil cooler to drain all water out. put plugs back in after it is drained.
4. Clean the flame arrester on the carb.
5. Add fuel stabilizer to the gas tank ( follow directions on bottle)
6. Purchase 5 gallons of RV antifreeze to put into engine.
a. pull water intake hose, use funnel to pour RV antifreeze through hose while engine is running. This will fill the engine with antifreeze that will prevent from freezing and cracking your block or manifolds and it has a rust preventitive. After all 5 gallons have been poured in the hose, shut off engine and reattach the hose you pulled off.
7. Ensure the transom or hull plugs have been removed to prevent any water from sitting in the bilge and freezing.
8. If you have ballast tanks you will need to empty those tanks and put one gallon of RV antifreeze in each of those tanks.
9. If you have a heater or shower system on the boat, pull the hoses off and blow them out with compressed air.
Some of this will require that you have some knowledge of engines and the different systems on your boat and some of it will require some tools or equipment that most people do not have at home. Consider all of this prior to winterizing your boat and make a decision on takig in to a dealer and spending roughly $300 to get it done properly or spending the money to purchase this equipment for yourself.