The iliotibial band (ITB) is on the outer aspect of the knee. This video shows how to do an ITB stretch.
In case you cannot see the video, here is a still sequence -
|The left leg is the one being stretched. Stand comfortably next to a wall, and support yourself with one hand on the wall.|
|Keep your weight on the right leg while you stretch your left leg behind it.|
|Bend the right leg (in front) to apply stretch to the left leg (at the back). Hold the stretch for a few seconds.|
|Bring the legs back to the starting position.|
|To stretch the other leg you will need to turn round and face the other way.|