The only steps one should really take to winterize an RPZ (the above ground reduced pressure zone backflow prevent) should be to turn on the lowest zone (preferably one that can be seen). Turning on the lowest irrigation zone from the system controller will start moving water through the backflow preventer, then turning off the isolation valve below the backflow preventer will help drain the valve through the zone that is turned on.
The zone will slowly shut off while it’s draining the irrigation system.
Don’t turn off the two valve handles on the backflow preventer because that will trap water in the valve and can cause damage if the valve freezes.
Turn off the isolation valve associated with the irrigation system as that will help drain the valve. This is usually found in the ground on the inlet side of the RPZ.
Next take the caps off the four test cocks and open them up. This will cause the RPZ to dump whatever water is left.
Leave the test cocks open as the rest of the water that’s inside the valve should evaporate and, even if it’s freezing out, there shouldn’t be any water to freeze inside the valve to break it.