1 - Turn off the shut-off valve - On most residential backflow devices, there are two shut-off valves. These are typically covered in blue rubber and are located before and after the actual backflow device. You need to shut the one off before the device (#2 on the picture above) to keep more water from entering it. This is the lower of the two shut off valves. Turning it a half turn will turn the water flow to the backflow device off
2- Release the water pressure - Now that you have shut off the water valve, you must release the water that is in the device so that it doesn't expand when it freezes and crack the device. The way to do this is, with a flat head screwdriver, loosen the two bleeder valves (#3 and #4 above) that are usually located just under the plastic top of the backflow. Sometimes, they may even have rubber inserts you have to remove before you can see the flathead screw. When opening these, be aware that the water will spew out for a few seconds and your hands will get wet.
NOTE: If the water doesn't stop spitting out after a couple of minutes, you haven't fully turned off the water in step 1.
3- Leave the smaller bleeder valves open - This will allow the water to expand and freeze without being hindered and causing damage.
4- Insulate your Backflow - At almost all home improvements stores, they carry pipe insulation. All you need is that and duct tape, and you can wrap the pipe as shown in the picture above. You'll also want heavy-duty scissors to cut the insulation around the bleeder and shut-off valves for easy access.
5- Turn Your water back on - Don't forget after the freezing temperatures have passed to go back out and close the bleeder valves, turn on the shut-off valve