Measure the perimeter of your return air ductwork. The perimeter is the distance around the outside of it. So if your ductwork is 16 inches wide by 8 inches high, the perimeter will be 48 inches. Add around 6 inches to this measurement to allow for your insulation. This figure is your cut length.
Stretch out your roll of duct insulation on a smooth, hard floor. Cut your insulation at the above calculated measurement with a sharp utility knife. Using the above example, if you had a perimeter of 48 inches and added 6 inches to that, you would cut your insulation at 54 inches.
Place your insulation around your ductwork with the insulation side facing toward the ductwork and the foil vapor barrier facing outward. It's easiest to drape it over the top of the ductwork and allow gravity to hold it in place momentarily.
Tape one of the ends of the insulation to the duct with foil tape. Stretch the insulation tight and tape the other end down with foil tape. This end will end up being taped down to the insulation jacket. Ensure tape runs along the entire seam to seal it properly.
Repeat Steps 1 through 4 until your ductwork is completely insulated.
Tape each seam where the separate insulation pieces butt together with your foil tape.