Sit on the ground with your knees bent and your heels about a foot from your butt.
Lean slightly back without rounding your spine at all. It is really important, and difficult, to keep your back straight, but don't let it curve.
Place your arms straight out in front of you with your hands one on top of the other. Your hands should be level with the bottom of your ribcage.
Pull your navel to your spine and twist slowly to the left. The movement is not large and comes from the ribs rotating, not from your arms swinging. Inhale through center and rotate to the right. This completes one rep.
To make this exercise more challenging, hold a medicine ball or a dumbbell in your hands with your elbows slightly bent. You can also make the move more advanced by lifting your feet three or four inches off the floor.