Stand tall, one handle of the chest expander fixed at your hip, with the arm straight, the chest expander running diagonal over your back. The other arm is pushing the chest expander straight overhead. Doing a slight curved outside motion can make it less stressful for your shoulder. Perform as many repetitions as possible, then switch hands.
The Shoulder Press is a great exercise to help you with your overhead pressing power, because it includes your upper back. Your upper back needs to control and stabilize the resistance of the chest expander.