Place the feet of a sturdy ladder securely on flat ground next to an exterior wall, and lean the ladder against the structure. Climb the ladder, and go up onto the roof.
Install a flexible rod, included in the dryer vent cleaning kit, into a cordless drill. There are normally four of these connectible 36-inch rods that, when all connected and installed in the drill, give you 12 feet of reach.
Install the kit's lint brush on the end of the rod.
Hold the drill in one hand. Grip the end of the last rod, just behind the lint brush, with your other hand. Insert the brush down into the rooftop dryer vent.
Squeeze the trigger of the cordless drill, allowing the rotating brush to clean out the lint stuck inside the vent. Pull the drill back, and push it back down repeatedly, to break up the lint and push it up and out of the vent. Pull the drill away from the vent to pull the rod and brush out of the vent.
Scrub the flapper and its hinges inside the vent, using an old toothbrush. This flapper prevents backdrafts into the clothes dryer.
Turn on the dryer for several seconds so the air from it blows any remaining dislodged lint up and out of the vent.