Use either John Guest 28 mm Nylon Pipe or 28 mm Copper Pipe. The pipe to be cut square and be free of burrs.
Press the shorter spigot into the pipe. The fit on Copper Pipe will be loose, this will not affect the function.
Push the pipe and convertor up to the pipe stop of the centre leg of the tee.
Turn the screw cap approx 1/4 turn to lock the pipe in position.