Step 1
Select a lightweight plastic or thin aluminum vent cover. Vents come in a variety of standard sizes and materials. The less it weighs, the better your chances of successfully installing it into the drywall.
Step 2
Hold the flat side of your vent against the ceiling, aligning it with the duct. Use a pencil to mark the holes on each end. Your pencil marks should be at least ½ inch from the edges of the drywall edges surrounding the duct opening. If not, you'll need a larger vent.
Step 3
Fit your drill with a bit slightly smaller than the diameter of the anchors, and drill a hole into the ceiling drywall at each pencil mark. Position the drill so the bit stays straight, not slanted to either side.
Step 4
Push the plastic part of the drywall anchor into each hole. A drywall anchor has two parts: a split plastic anchor with ribbing, which contracts slightly when inserted into a hole, and a threaded screw that fits inside the plastic anchor.
Step 5
Hold the vent cover in place, aligning the holes in the vent with the holes you drilled into the ceiling.
Step 6
Use a screwdriver to insert a threaded screw into each plastic anchor while holding the vent in place. Tighten the screw just until it is snug against the vent cover, but do not overtighten. As you insert the screw, the plastic anchor will expand outward, hugging the drywall.