Cutting into duct for 2nd floor vent from 1st floor
I am re-finishing a 10x10 room on the first floor and adding insulation to the exterior-facing wall. This area used to be part of the garage so there is no ventilation and the air gets very stagnant. I have cut a 13"x6" hole in the ceiling for a register that I bought, and would like to cut into the duct that goes between the two floors that supplies the room above (which is the smallest bedroom in the house).
What is the best way to go about doing this beyond just cutting a hole into the duct? What can I do and/or purchase to make the installation respectable without affecting the upstairs room very much?