The first thing to do is to determine if the system uses auto-drain valves. If auto-drain valves are present, then all you need to do is turn the main water supply off and drain the lines. The water should flow down hill and out of each zone through the auto drain valve.
If a valve is present open it to allow all water to drain.
If your system does not use auto-drain valves, then an air compressor must be used to blow the water out of the zones. Turn off the main water supply to the sprinkler system, and hook up an air compressor to the system. You have to pressurize the main tank of the system and then open each one of the valves manually. Let the compressor run through each valve for two or three minutes to make sure that all the water is pushed out of the line. Shut that valve off, pressurize the main tank again and then repeat for each head.
Winterizing your system isn’t something that you really want to mess around with however and if you have any question at all as to how to complete the process it’s very important that you contact a professional to do the job for you.