I just completed my 10th winterization of IP35-165, Island Spirit. Here is a summary of the steps.
Pump water tank empty
Bypass water heater (connect hoses together) drain tank
1 gal into engine intake, pump to head
1 gal into engine, run until exhausts shows antifreeze
1 gal into sink, hot, cold and sprayer
1 gal into head sink and shower lines
1 gal into the AC/heat system
1 gal into deck wash & cockpit shower
1 gal for shower bilge, and bilge
To add antifreeze into the water lines, I disconnect the pump pick up line at the tank and connect it to another hose about 2-3 feet long. I then put this hose into the 1 gallon jug and turn on the pump. Using a white bowl, I run the water at each location until I see pink then stop.
It is so easy, and it takes about 1-2 hrs to winterize the boat. We never put antifreeze into the tank, and we never run it into the water heater.