This is an exercise I learned from my gymnastics teacher.  If you can do a wall walk to start, try getting into the basic backbend or wheel as they call it in yoga.  From there lift one hand for five seconds, put it back.  Lift the other hand for five seconds, put it back.  Then lift a leg, then the other leg.  Try to remain fully inflated, and stretch through your upper back as much as possible.  You want the pressure on your arms and shoulders, not your lower back.  Good luck!