To perform a band leg curl, choke a band around machine or stationary object at about ankle level. Then take a bench and put it about 2 feet away from the machine. Take the end of the band that isn't attached to the machine and place it over your ankle so that it wraps around your ankle. Sit down on the bench and push it back some so that the band has constant tension on your leg. While sitting simply curl your hamstring and pull you foot back towards the bench you are sitting on.
A beginner lifter could easily start with a purple/light Jumpstretch band and a more advanced lifter could move up to a green/average band or even a blue/strong band. Aim for higher reps on this exercise. 10-25 is best. It is also great for working around knee injuries when you can't squat or deadlift.