If you use metal shears to cut thin sheet metal such as duct metal, it curls and can be difficult to flaten properly. These photo-steps show that it can be done using a utility knife. The illustrations are for a small bracket, but I have cut 3 foot long pieces using this method. You just need to be careful and take your time making several scoring lines and not pressing too hard. Snap off a piece of blade as necessary and don't use the thinest blades or the extra heavy duty types.