When the shocks are off the car you compress them as much as possible and then rotate the top relative to the bottom. You may be able to do it on the car by disconnecting the bottom end and then compressing the shocker and turning it.
Undo the lower bolt on the rear damper and slide it out of the damper lower eye. You then need to push up on the damper and turn it until you feel it engage in the adjuster. It's then just a matter of turning the damper whilst still pushing upwards to keep it engaged.