Sit on the ground with your knees bent and your heels about a foot from your bum.
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.
Hold a weight or a medicine ball just below your chest. Keep the weight close you and progress by moving weight further away from your body.
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 centre and rotate to the right. This completes one rep.