Measure the size of each of your vents with a tape measure (measure each one since the vents throughout your home may differ in size). Write down the measurements of each vent and label each measurement with the location of the vent.
Measure out and draw an outline of the vents on the magnetic covers using measuring tape and a pencil. Cut the magnetic vent covers along the outline with scissors. Since these covers stay in place with their magnetic connections, any extra, unused length or width hanging over the metal portions on the vent becomes extra unsupported weight that causes the cover to have problems staying attached to the vent.
Turn the vents' air flow off and place the covers over the vents. This makes it easier to place the covers over the vents so you do not have to fight the blowing air. Turn the air flow back on. Your vents are now covered from dust.