Stand with Neutral Spine, Butterfly and Core engaged.
Raise one arm forwards, with palm upwards. Continue raising until arm is vertical, next to your ear, then keeping the elbow raised, drop the hand behind your back. Use your other arm to aid the stretch.
Hold a gentle stretch for 10-15 seconds, and then when the muscles relax stretch further while you try to walk your fingers down your spine.
Hold for 10-15 seconds more, then release the stretch and repeat with the other arm.