To winterize a boat, you will need to go through many steps. One of the main things is to protect the engine. Treat the fuel and engine with a stabilizer, change the oil, drain the engine, and fill the block and circulating pump with antifreeze. If possible, adjust the engine to vertical position, making sure all the water drains out. Disconnect the fuel line from the engine. There are many other more detailed steps recommended by experts, such as applying an aerosol fogging solution to the engine.
Clean and dry the bilges. If you have a head and/or a fresh water system, empty and sterilize the head, drain the water system, and pump non-toxic antifreeze through it. Remove the head altogether if it’s removable.
If you are dry storing the boat, pressure wash and inspect the hull. Remove any barnacles, and patch up blisters. Then, get it covered with the best kind of cover or wrap. If it’s staying in a marina, many owners will install a dehumidifier, and in the slip they’ll install a bubbler that keeps the water from freezing solid around it.