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 the weight (dumbbell, kettlebell, med ball) at chest level.
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 advance this exercise, raise your feet off the floor.