Measure the dimensions of your air vents using measuring tape.
Purchase a package of appropriate-size magnetic vent covers. These are available at most home improvement stores; they come in quantities of 1 to 5. Be aware that, often, vent covers don't match the exact size of your vents. If this is the case, select a size that is larger than your vent.
Mark the appropriate width and height, using measuring tape and a pencil, on a magnetic vent cover.
Cut out the marked area using a pair of scissors.
Place the magnetic vent cover on your vent.