I've cut a few supply and cold air returns into my main branches and it's been a PITA. Need to cut another 6" hole for a return, it's just standard rectangle ductwork, probably 14" wide x 10" high.
Cut along your line all the way to the corner, with force turn and continue while gently pulling up on the scrap. Every 2-3 inches turn in off the line and make a relief cut and go back to the line. Once you get to the next corner you don't need relief cuts anymore.
COMPLETE THE CUT Now it's easy, just cut along the line gently pulling up the scrap. If you are left handed, or only have greens feel free to use Greens but reverse the images. Yellows are designed to cut straight but are capable of cutting to the left like Reds
FOR CIRCULAR HOLES
The same process applies just without the relief cuts.
You might be able to rent a tool from a rental yard called a "nibbler"; you would start with a drilled hole and then the nibbler takes little semi-circular or rectangular bites out of the sheet metal. There are also inexpensive ones now that attach to a drill motor, one is shown in this video. He has troubles with it because he is trying to use it on corrugated sheet metal, you would not have that issue. https://www.youtube.com/watch?v=Wt4O62qnHsk