To connect new ductwork to existing ductwork, identify points in the old ductwork where you plan to install new outlets, plan a direct route between those points, and connect the points with a metal sheet collar. Make sure the new route is free from any obstacles or hazards.
Integrate a metal sheet collar into the main duct trunk by cutting a hole equal to the diameter of the collar into the trunk. Push the tabbed portion of the collar into the duct, and secure it by folding the tabs inward. Push the other end of the collar into the new duct, and screw it in place.
Add wood blocking under the duct in the ceiling joist space. Cut the wood blocking with dimensions equivalent to the width of the ceiling joist, and then slide it up and nail it in place. Place the wood blocking at every 5 feet of the duct so as to provide constant support for the load.
Install a new return duct with the same capacity as the new supply duct. Balance the ductwork so that the return system collects all the air that is released by the unit. Fasten the return ducts with adjustable tape and straps.