Attach a 2-by-2-foot plank over the furnace ducts and about 2 to 3 inches in front of the ducts. Fasten the nail strip to the side of the plank with the nail gun and nail the other side of the strip to the perpendicular wall stud to hold it in place.
Attach a 2-by-2-foot plank under the ductwork and parallel to the top plank. Use the nail strip and nail gun to attach it to the wall stud in the same way.
Nail 2-by-2-foot studs between the planks every 16 inches. Nail more 2-by-2-foot studs between the bottom of the soffit, located under the furnace ducts, and the wall next to the ducts. Nail the planks to the studs in the wall.