The best way to install a backflow valve is to shut off water supply, remove the strainer, and to install the backflow valve in the floor drain or backflow pipe. Tighten the screws that hold the valve in place, or use a pressure washer to seal the supply pipe.
Locate the pipe in which the backflow valve is to be installed. When using a threaded and glued backflow valve, drop a threaded T device into the pipe and connect it to the pressure valve adaptor. For compression fitting backflow valves, cut a piece of the line to install a compression device and set a pressure washer to seal the outside of the supply pipe.