Slide the rotor back into the valve casing, so that the screw on the bottom of the rotor peaks through the hole in the bottom of the valve casing. You may need to use a rubber hammer to hammer the valve back in place. This is a delicate process. Take your time and use light strokes until the valve is back in place.
Align the valve by turning it until the notches on the side of the valve line up with the alignment indicator inside the valve. The valve will drop into place when the alignment is correct.
Screw the bottom nut back on about three turns clockwise, until the nut is flush with the bottom of the valve casing. You should not need tools for this process.
Replace the top valve cover and turn in clockwise until the valve cap fits securely.