1). If the boat is still in the water, warm up the engine and change the oil and the oil filter.
2). Drain as much water from the potable water system as possible using the pump. If you like, you can use a shop vac to suck the dregs of water from your potable water tanks.
3.) Blow as much water from your water heater as possible using the drain plug in the bottom, and the water inlet hose. Connect the inlet hose to the outlet hose, bypassing the heater for the next step.
4.) Pour some of the pink or the purple (-50 or -60 deg) RV/Marine antifreeze into one of the potable water storage tanks (If you have more than one tank, evaluate the pipe runs to determine if you need to run the antifreeze from all of these sources), turn on the pressure water pump and open the hot water tap that is furthest from the pump. When pink starts to come from the tap, close the hot and open the cold till it runs pink too. Repeat for all of the taps, working your way back to finish with the taps closest to the pump.
5). Drain the shower sump and the ice box and run some of the aitifreeze through those lines.
6). Once the engine isn't going to be used again, close the engine intake through-hull (that is, if your boat is still in the water, otherwise, you can leave this open)and disconnect the hose between the water stariner and the raw water pump. If possible, stick the end of this hose into the top of a gallon bottle of the RV/Marine antifreeze and turn on the engine. Run the engine until pink (or purple) antifreeze comes out the exhaust (it should take less than one gallon, but it may take more, so it helps to have a second person topside to watch the exhaust and turn the engine off when appropriate).
If your engine is raw water cooled (that is, sea water goes directly through the engine and there is no secondary cooling loop), life becomes more difficult as you MUST run the engine long enough to get the thermostat to open before you start with the antifreeze (the small Universal (11 HP) uses raw water cooling, but there may be others in the Catalina fleet).
7). Make sure that the head is pumped out, then perform essentially the same task with the head that you just did with the engine (that is, remove the hose from the through-hull and suck antifreeze through the pump, into the head, and from there to the holding tank.
From here, things are unlikely to break, so how you handle the cushions (I remove mine), ventilation (I make sure that all compartments are open and that I hang mildew packets throughout the boat), electronics, other gear, sheets, haliards, and covering the boat (a very good idea) is up to you. As for storage with the mast up or down, there are both pros and cons to both ways, but in Michigan, most of the boats (outside, cold storage) are stored mast up.
Best of luck at a terrible time of year.
Steve
Alchemist C-380 #71
Alchemist C-320 #909
Alchemist C-30T #4764
Dividend Cal 27 #99