1. Turn the water off to your sprinkler system. Usually in your basement or crawl space.
2. Go outside and turn all of the ball valves on your Backflow to a 45 degree angle as pictured below. Make sure you do this to all 4 ball valves. Two should have big handles like the blue handles below and two are the little flat head screwdriver slotted test cock valves that stick out of the side, sometimes they are in different locations depending on which brand of backflow you have.
From this.
To this.
3. Open up your valve box/es and open the drain/s. Valve boxes are in the ground and usually green. Drain valves are usually located at the end of the line past the last zone valve. Most of them have a cross shaped handle, but some of them are ball valves similar to the valves on your back flow and can be plastic. By opening your drain valve/s in the valve box/es, the water will drain off of the front side of your backflow and copper pipe exposed to the air, and will drain your mainline that connects to your valves. Pictured below, drain valve is on the right bottom of the picture (cross handle).
4. Bring a large cup, bowl or bucket and go back to your basement or crawl space and open the drain to drain the back side of your backflow and pipe. These drains are usually attached to the valve that you shut off the system with. Pictured below. They are usually a little tiny cap that unscrews to let the water out. But it is very important to do steps 1-3 before opening this drain cap or you willl not get all of the water out. When done draining, leave this cap loose or unscrewed with a bucket underneath to catch any dripping if possible.