Start lying on the floor on your back and activate your lower abdominals by contracting your abs and pressing your low back into the ground. Then lift both legs in the air and move them together in a circular path. The quality of the circle doesn’t really matter, as long as you keep a controlled motion throughout the exercise.
This exercise can be used with a wide range of ability levels, because it can always be made easier or tougher. The more bend you have in your knees, the easier the exercise and the closer your legs are to being straight, the tougher it gets. Additionally, the larger and lower the circular motion, the tougher it becomes.
That said, the goal is not to see how big you can make the circle or how low you can go, but rather how difficult you can make the movement while still contracting your abs correctly.
Your low back will probably not stay tightly pressed to the ground, but it shouldn’t arch or curve much. Making the effort to keep your back flat is where the majority of the benefit comes from with this exercise.
If you can’t keep your back from arching significantly, it means the exercise is too difficult for your abs to stabilize correctly. If this happens, either bend your knees more, raise your legs higher, or decrease the size of your circle until you can do the exercise with good form.