To perform band pulldowns it requires at least one band or two bands. To setup band pulldowns with one band, take the band and choke it around the one of the frame members along the top of the rack. You will probably start with a purple band and work up from there to a green band or blue band.
The band should be hanging perpendicular to the floor at this point. Sit down on the ground below the band and grab the band with both hands and pull down with your elbows down to the floor. You can wrap your hands around the bands once to increase the resistance of the band a bit. You can also widen your grip on the band to bring the rear deltoids more into the exercise.
By using a band for this exercise you get a lot more bang for your buck instead of a regular pulldown using a cable machine with a fixed bar. Depending on your bodyweight and strength level you may need to weigh yourself down to the floor by putting a plate on your lap so you don't end up doing band pull-ups.