I am installing an Aprilaire 700 series humidifier on my heating system. I have the template for the size of the opening on the supply duct. How do I cut the opening on the plenum and what tools do I need?

Aviators, commonly called snips are used. One red and one green handled will cut left hand or right hand radius', I suggest you buy both. Start a hole with a large drill bit, uni-bit or HVAC guys usually just use a hammer and old standard screw driver to tear open a hole in the middle of what you want to cut out.

just beware ,sometimes inside that plenum,that is where your a-coil,is ,i`ve seen it several times,you don`t want to punch a hole into that baby.