Rest your back on a wall, with your feet a step away from the wall.
Engaged your Abs and use a posterior Pelvic tilt position.
Lift your arms up, with the elbows bent, and push them back against the wall, or as close to the wall as possible.
Slowly extend your arms overhead, making sure to keep your hands touching the wall, or as close as possible.
Form tips
It is important that you keep your lower back in contact with the wall for this exercise to be effective. Arching the lower back would change the angle at the shoulders, thus making the exercise easier to perform, and progress harder to see.
If you cannot keep contact with the wall with your hands at all times, that is ok. You can hold a small object to touch the wall and track your progress over time, by reducing the length of the object.