First check to ensure that all 'plugs' are installed in the water system. This may include some of the following 'low point drains' for hot and cold water lines, fresh water tank drain and hot water tank drain.
'Winterizing' generally refers primarily to filling the water lines with antifreeze. So you want to hook up to a hose (use a pressure regulator to avoid causing any damage inside the tt) and run every faucet (including the showers, inside and out) and the toilet until all the antifreeze is flushed out. Don't forget the washer if there is one installed.
Check the water heater and see if it has a 'bypass valve'. If so, set it to 'normal' mode (so the hot water tank will fill).
Check the water and charge of the batteries, the condition and pressure in the tires.
Fill the fresh water tank to the desired level (a good inline filter can help keep it clean). Load it with toilet paper, your chemicals of choice, hookup stuff (cords, adapters, hoses, etc), clothes, food, personal stuff, toys, etc.