If your adding DUCTwork to the basement, you tap into the supply trunk. It’s called a takeoff. Malco makes a circular cut out tool, that attaches to your cordless drill, and cuts a perfect circle, to a preset diameter. Then you can attach a starter collar and run ridgid pipe or flex to the desired locations. You should use the starter collars that have pre-installed dampers in the basement, so the air flow can be adjusted.