Lie on one side with your ankles overlapping and your body bent slightly at the waist for balance so you will not tip over. Rest your lower hand on your waist while your other hand is behind your ear like for a crunch (your elbow should be toward the ceiling).
Now, try to pull your ribs toward your hip by engaging your oblique. While doing this you should be pulling your ankles and knees as well as your shoulder you are lying on off of the ground. Use your hand that is on your side to feel for your oblique tightening.
This is an advanced exercise. Beginners may need to place some weight on their elbow and use it as a pivot point. It may also take some time before being able to pull your knees from the ground, but until then, squeeze your obliques as tight as you can to take as much weight off your knees as possible.