First thing I'd so is have the backflow preventer certified. When a sprinkler system is winterized, it is usually blown dry with compressed air. The pro use an industrial compressor and can blow the whole system at once. I also can be done with a small compressor one zone at a time. They simply manually open the zone valves and apply air. To restart, all that is necessary is to turn the water supply valve on and close any hose bibs that are not run through the valve system. After that, just turn the control box to ON and you're in business. Some places do not enforce the EPA requirement on back flow devices, others, like mine, require annual inspection and certification. Whether the laws are enforced or not, this should be done to avoid the possible contamination of your water supply. There should be at least one business in your town that deals with sprinkler systems and can advise you on your brand of controller, and who is a certified backflow inspector in your town. I am appalled at how many local towns do not comply with EPA regulations on cross contamination prevention.