I was thinking of blowing out my own sprinklers this year because it seemed pretty straight forward and I am handy enough to do it. I get the basics: shut off main water, connect air compressor to system, open one zone at a time. Here is a picture of my backflow system. From what I would assume, I would turn off the valve to the right (probably the main shutoff), keep the valve on the left open and make an air compressor fitting for the pipe coming off of it that is capped currently. Does this seem correct? This is the box the sprinkler people go to for spring and winter maintenance.
Your plan seems reasonable.
Some things to note:
- The four little stubs sticking off the bottom are valves - probably ball valves. They could have water trapped inside the ball, so you'll want to blow those out and/or leave them open when you're finished to prevent the valve itself freezing.
- The main valve is also a ball and will have water inside it too. There should be another valve somewhere in a freeze-proof location -- either inside the house or a few feet below ground out in the yard. It's find to close the right-hand valve here while doing the blow-out, but you'll want to leave the upstream valve closed and this one open when the job is finished.