In sitting, find pelvic neutral by sitting up as straight as you can and then relaxing down a little to the point where your back muscles are relaxed.
Engage your pelvic floor and deep transverse abdominals.
Gently engage your shoulder blade muscles, lower trapezius by widening at the collar bones.
Supporting yourself through one hand, take the other arm above your head, keeping your shoulder blades engaged and stretch over towards the supporting arm. Re-engage your shoulder blades and you should feel a stretch up the side of your back, on the side you are stretching away from. Hold, then relax. Repeat on the other side.