Measure the vent and register to make sure they are the same size. Test the damper by flipping the damper lever back and forth to make sure it opens and closes. Do not install a damper that does not work properly.
Hold your other hand beneath the ceiling vent to catch it after the screws have been removed. Unscrew the two retaining screws from the register.
Push any insulation aside so that the hole is clear of any obstruction.
Hold a register up to the hole so that the damper is in the ceiling and the grille of the vent is flat against the ceiling.
Drive a retaining screw into each of the screw holes at the ends of the register until the register is tight against the ceiling.
Retest with the damper lever to make sure the damper opens and closes properly.