What's the best tool for cutting holes into duct work?
STEP ONE
Using a big flat screwdriver orienteded edgewise and a hammer. Strike the screwdriver swiftly and accurately to puncture the duct.

STEP TWO
Insert a Red pair of snips into the hole and nibble around turning to the left (Greens are pictured only because that's what was in front of me) in an expanding circle until you get to the line.

STEP THREE
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.