Select a rope that is at least three times longer than the longest duct you want to clean. Unroll the rope and locate the middle of it.
Soak a towel in household bleach and wring it out so it isn't dripping wet. Wrap the towel around the middle of the rope and secure it with a rubber band.
Have a helper stand at one opening of the first duct, while you stand at the other opening. Feed the end of the rope into the opening of the duct until the end reaches your helper. Have the helper pull the rope farther so the towel goes into the pipe.
Alternate pulling the rope with your helper. Pull the towel from one end of the duct to the other. Rinse the towel in water each time it comes back to you and re-soak it before the helper pulls it back.
Repeat this five times, then remove the towel, soak a new one in the bleach, and attach it to the middle of the rope. Repeat the process with a new towel for each smelly duct.
Run the furnace for 20 minutes when you are done. This will dry the inside of the ducts.