Begin by measuring and marking space on the ceiling for the new vent. Steps for vent installation then include making a vent space, mounting and securing the vent boot and register box, sealing space between the vent boot and drywall and connecting the vent to the duct trunk.
With a finishing nail, make a few small holes to ensure the vent space will rest safely in between joists. Using a spiral saw, cut the drywall to make the vent space. In the attic above, construct a simple wood frame and use it to mount the vent boot and register box. Secure the flex duct liner to the register boot with metal foil tape. Pull the flex duct insulation jacket over the vent boot and fasten it in place with a zip tie. Secure the register box to the wooden frame with 1/2-inch sheet metal screws.
Carefully seal the space between the vent boot and the drywall to prevent leakage and energy waste; spray foam, caulk or mastic may be used for the job. To make the new vent operational, connect it to the rigid duct trunk using flex duct and a start collar by cutting a hole in the duct trunk and securing the start collar with metal foil tape. Pull the flex duct's insulation jacket over the start collar and fasten it with an additional zip tie. Screws are then used to attach a register to the new vent.