Measure the vent opening with a measuring tape so you can determine what size cover to get.
Purchase a vent cover that fits the vent. Choose either a heat register cover or a cold air vent cover, depending on the type of vent you are covering. Cold air vent covers never have dampers on them and help the air circulate in the room. Heat registers usually have dampers. You can also find vent covers that function as both heat register covers and cold air vent covers. Select a color and design that suits your decor.
Drill pilot holes around the edge of a wood vent cover where your screws will go. If you are using a metal vent cover, buy one that is predrilled. For small, standard covers, drill one pilot hole on each end of the cover. Larger covers need a hole on each side, plus two holes on each length if they quite large and heavy.
Install the cover into the ceiling around the vent by screwing the appropriate screws through the pilot holes and into the ceiling. If the ceiling is made from drywall, use drywall screws.