To "winterize" means to prepare the home for when it's unoccupied.
The first step is to locate the water shut-off valve, preferably one that is outside.
With the water turned off, it's now time to turn off the water heater. A gas water heater should be shut off at the gas valve. An electric water heater needs to be shut off at the breaker.
Now that the water heater is turned off, drain the water. Open the faucets at the fixtures, and then go outside and open the outside sillcocks. Gravity will drain the water out of the heater.
A compressor adjusted to 40 psi's (Image 1) is needed to prevent the air pressure from bursting the pipes. To connect the compressor to the outside sillcock, build an adaptor.
Start by using a standard sillcock and wrapping the threads with Teflon tape.
Add a 3/4" hose thread adaptor and a 1/2" to 3/8" coupler.
Use a pair of slip-joint pliers and an adjustable wrench to make sure all the connections are tight (Image 2).
Add Teflon tape to the threaded end of the compressor hose, and connect the adaptor. Use a wrench to secure the connection.
Connect the hose to the compressor, and then use a washing machine hose to bridge the adaptor to the outside sillcock on the house. Hook it up to the system and blow out the water that may be stuck in any low spots so the entire system is free of water.
Note: This is the same process for winterizing an outdoor sprinkler system.
Now that the valves are closed in the house and the valves outside are open, it's time to pressurize the system. Turn on the compressor. If the pressure won't build up, that means there's still an open valve or a broken pipe.
With the system charged with air, the water heater can quickly drain. The air will come into the cold water line and rise to the top of the tank and actually push all the water out. This water will come out quickly so be sure to let it cool off for a while before draining.
The connection to attach a garden hose to is called the "draw-off." They're usually located at the bottom of the tank. Some turn by hand, others will need a screwdriver to open (Image 1). Once this drains, move on to the other fixtures.
Start by opening up the farthest outside sillcock from the compressor. When only air is coming out, close the valve and move onto the faucets inside the house.
Go to each faucet and open them up (Image 2) until only air is coming out. Close the faucet tightly and move on to the next one.