Gas and Oil Home Heating Furnaces - Installing a heat register?

Hello,
I'm finishing out a basement and need to bring heat into the new bedroom I built. There are 8" heat ducts above, and I have a suspended ceiling sheeted with drywall that will be installed. Do I simply cut a hole in the duct and attach say a 4" duct and bring down and attach to the ceiling grid. Then once the drywall is up, attach the register?
Any help is greatly appreciated!

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Tom_B

I would go with a 5" or 6" duct for better air flow, and if you want a rectangular ceiling register as opposed to a round one ( i think rect. looks nicer) you'll need a register boot, which goes from round to rectangular for the register to slide into a 2 1/2" x 12" or a 4 x 8 should work nicely

What are the existing ducts feeding- you may rob those froms of air flow