This is for plain, old-fashioned Mettle ducting, that is unlined.
*If you have Duct Board, Which is fiberglass, or Flex Duct, then you can NOT clean it yourself. These have lining, and other fragile components. You must get a pro. You can damage it beyond repair.
-Bucket or Bin, to collect debree
-Small brush or broom
-Paper towels or Rags
-Vacuum with HEPA filter
Step 1: Step 1. the Main Hatch
Locate the Draft Control Valve.
It should swing open easily.
Step 2: Step 2. Brush It Out
Using, a dry paper towel, or a small dry broom, Brush the dust and grit out of the conduit.
* It may be tempting to get the rag damp, but Don't do it! It will just make mud, and then the residual moisture will cause mold.
Step 3: Step 3. Vacume It Out
Once you have the bulk out, stick that vacuum in there. If you have a flexible hose on your vacuum, all the better.
Step 4: Step 4. Registers
There are two kinds of Registers, Supply and return. Supply is where the warm are comes into a room, and return is where air gets pulled into the furnace, to be heated.
The registers, or vents, that can be removed/opened, should be sweep, then vacuum what you can reach.
Step 5: Step 4.2
After you have gotten all of the registers in the main areas, be sure to get the out of the way places. This goes for the Basement ones as well. These are often the best, as they are located on straight pieces of duct, allowing better reach.
Step 6: Step 5. Enjoy
You have done as much as you can; aside from dismanteling and reassembling your ducts. *Which we do NOT recommend.
Now you can enjoy cleaner air, and a sense of accomplishment.