How do I start a 9" inch flex duct run from a 16 x 14 plenum with an open space at the end?

I am doing the duct work for my new office space. I've got everything down except for the takeoff from the plenum and the cold air return. The furnace already has the plenum attached and has a 16" x 14" rectangular opening above the drop ceiling. I figured I would need to seal the opening with sheet metal, then cut the correct size into the sealed opening (9" for the flex duct), then insert a take-off collar into the opening and attach it from there. Is this accurate? Would I do the same for the cold air return that goes back into the furnace?