Measure the height of the existing air vent opening from the floor. Measure both the bottom of the air vent and the top of the air vent. Measure the distance of the existing air vent opening from the nearest wall. Measure both the closest and farthest point from the wall. Record the measurements on a piece of paper.
Transfer the measurements onto piece of drywall that you will install over the existing air vent. Be sure to take into account the distance above the floor that you will mount the drywall -- usually 3/8 inch. Use your ruler as a straight edge to draw a rectangular box duplicating the opening onto the sheet of drywall.
Score the lines onto the drywall with a utility knife. Insert a keyhole saw into the lines and cut the opening out of the sheet of drywall.
Place your sheet of drywall onto the wall, positioning the opening over the air vent. Screw or nail the sheet into place.
Hold your air vent cover in position over the opening. Mark the two screw holes in the air vent cover onto the drywall with a pencil.
Drill a hole at each of the marked locations and insert a hollow wall anchor into the drywall.
Slide the anchor's screws into the screw hole openings on the air vent cover. Position the cover over the vent. Insert the screws into the hollow wall anchors and tighten them until the vent is firmly in place against the drywall.