How to install a/c vent to existing duct work.

I am finishing a part of our basement for our boys to use as a bedroom. There are two ac/heater ducts that run in the floor joist above the room. They are round aluminum. I am placing a drop ceiling in approx. 8 inches below those ducts. How do I put a vent in each of those ducts pointing down into the new room. I can't figure out how to do it with the round duct work that is already there.