Summarize the following:
If you've lived in your home for a while, you probably know where most of the ducts are. Take some time to locate all of them anyway in order to save time. Some homes have vents high on the walls, and others have them located at the base of the walls. Plan to clean each one of these in a systematic fashion. Basically, clean them room-by-room—unless you want to skip around to make it last longer. Some vents might be located in unusual places. Roam around your house to locate them all. You'll probably find other things that need to be cleaned or fixed in the process, but don't get distracted. Check for anything underneath the duct, such as furniture, hanging pictures, carpet or rugs, decorative items, etc. Move them out of the way or cover them so that they don't get dust or dirt landing on them. You don't need another thing to clean. A ladder, step-stool, stable chair, or particularly strong person that can hold you up will work. Make sure that the item is sitting flat and does not wobble under your weight. Cleaner air is great, but it doesn't have to come at the cost of a twisted ankle. In basic terms, the supply registers are the things that supply heated air to the rooms. Lift the register, cover it in a paper towel, and put it back in place. It's as easy as that. This will keep excess dust from being released into the room while you clean the air ducts. Doing this will keep everyone in the house from inhaling and coughing up all the dust you're about to release. Go to your thermostat and turn the fan on. Turn the heat and cool mode off. You'll only want the fan to be running. Look at your furnace filter to make sure it isn't missing or damaged. The filter is important so that the dust you're about to clean out doesn't get stuck in the fan motor. That would create another problem. Use something similar to a toilet brush, but you probably don't want to use the toilet brush you already have. A fairly large, stiff-bristle paintbrush is ideal. Use that brush to knock loose the stuck or damp dust in the air vents.

Summary:
Locate the air ducts in your home. Look underneath the ducts. Set up a stable surface to stand on if your vent is high. Cover the supply registers with paper towels. Turn on a fan in the room you're starting with. Check the filter. Loosen up the dust in the air ducts.