Step 1
Lay a sheet of duct board on a clean and flat surface with its fiberglass side facing up and the duct board's foil backer facing down. Duct board comes from the manufacturer in 48-inch-wide by 96-inch-long pieces. One of the 96-inch-long sides has a female shiplap edge. The other 96-inch-long side has a male shiplap edge with a flap.
Step 2
Lay the 24-inch side of the duct square on the 96-inch-long side of the duct board with the flap. The duct square's 48-inch side extends across the duct board's 48-inch-long side.
Step 3
Slide the duct square toward the left-hand side of the duct board until the correct measurement mark on the duct square's 24-inch side lines up with the left-hand 48-inch-long edge of the duct board. The first cut's correct measurement equals one half the thickness of the duct board plus the inside width of the finished duct. For example, a 24-inch-wide duct made out of 2-inch-thick duct board has a 25-inch long first cut.
Step 4
Hold the duct square still with one hand and push a red-handled duct knife along the duct square's 48-inch side. The edge of the red knife must touch the duct square's side. A red knife makes a 90-degree V-groove for the duct board's corners.
Step 5
Move the duct square to the right along the duct board's edge until the correct measurement mark on the duct square's 24-inch side lines up with the right-hand edge of the V-groove. The correct measurement mark for a duct board's second cut equals the height of the finished duct.
Step 6
Keep the duct square still with one hand and run the red-handled duct knife along the duct square's 48-inch side with the other hand.
Step 7
Slip the duct square along the duct board's edge until the correct measurement mark on the duct square's 24-inch side lines up with the right-hand edge of the second V-groove. The correct measurement mark for a duct board's third cut equals the width of the finished duct.
Step 8
Hold the duct square still with one hand. Push the red-handled duct knife along the duct square's 48-inch side with the other hand.
Step 9
Slide the duct square across the duct board's edge until the correct measurement mark on the duct square's 24-inch side lines up with the right-hand edge of the third V-groove. The correct measurement mark for a duct board's fourth cut equals the height of the finished duct.
Step 10
Press the duct square against the duct board's surface. Run a blue-handled duct knife along the duct square's 48-inch side. The blue knife creates a shiplap edge with a mounting flap.
Step 11
Remove the duct square and duct knives from the duct board's surface. Cut through the duct board's foil backer with a knife, using the blue knife's far right cut as a guide.