Locate a spot on the duct work that has enough space to accommodate a vent opening.
Place the vent base flush against the duct. Trace around the entire base of the vent to create cut layout lines.
Drill a 1/2-inch hole within the layout lines to accommodate the tip of the tin snips.
Open the tin snips' cutting teeth and place one inside the ½-inch hole. Squeezing the tin snips' handles repeatedly, cut the duct work following the traced lines.
Slip the base of the new vent into the hole. Press the outer lip of the vent flush to the duct. Secure the vent to the duct with ½-inch to 1-inch self-tapping metal screws.