Put on safety goggles and mask first. Ensure you are wearing clothes that do not expose too much flesh. It is advisable to wear gloves as well.
Measure and mark where you want to cut on the duct board using the measuring tape.
Place the metal ruler next to the mark you made and stick it down with double sided tape so that it won't slide.
Using the duct board knife, first score a line over where you want to cut so that it creates a groove in the board. As this piece of duct board is 1 inch thick, some patience will be required when cutting it. Apply as much pressure as necessary and keep the knife at a lower angle.