Clean areas around the vent that lead to the air duct. If your vent is located on the floor, vacuum or sweep around it first so no dirt falls in.
Remove the vent cover or grill. Once you have unscrewed the screws that hold it in place, it should be easy to lift off.
Reach into the air duct and wipe the sides with the rag or paper towels. This will clean the filters at the head of the air duct. Place used rags into a plastic bag until you can wash them. Wear gloves when you reach into the air duct.
Reach down into the air duct with your vacuum hose. Don't turn on your vacuum until you have moved the hose past the first filtered section of the air duct. Vacuum suction can damage filters in this area.
Vacuum the duct. Though you will not be able to reach all the way into the duct with your vacuum, you'll be able to clean out much of the dust and debris that has collected.
Return the grill to the air duct. Once you have screwed it back in place, vacuum the outside of the grill to remove any remaining dust particles.