Measure the neck of the register you wish to cut into the existing duct. The neck is the portion that contains the damper and fits into the duct.
Add 1/4 inch to the dimensions of the register neck and transfer the measurements to your duct at the desired location of the outlet with a marker.
Drill out each corner of the marked out rectangular shape with a 1/2-inch drill bit.
Place the jaws of your snips into one of the holes and cut along the line to the next hole. Continue cutting along the line and remove the rectangular piece of sheet metal once complete.
Fit the register into the opening. Level it and mark out the screw holes. Remove the register and set aside.
Drill out each screw hole with a 7/64-inch drill bit. Position the register back into the opening and align the screw holes.
Fasten the register to the duct with the provided screws. Adjust the damper as necessary to let more or less air into the space.