The method you choose to cut sheet metal depends on the thickness of the metal and the type of cut you plan to make.
Cut sheet metal without curling.
The illustrations are for a small bracket but i have cut 3 foot.
These photo steps show that it can be done using a utility knife.
Among others the most basic manual tool that you can use is a tin snip.
If you use metal shears to cut thin sheet metal such as duct metal it curls and can be difficult to flaten properly.
How to cut sheet metal manually.
How to cut thin sheet metal and keep it flat.
To cut straight lines use an electric saw.