Stand tall, one handle of the chest expander fixed at your hip, with the arm straight. The other arm is pushing the chest expander straight overhead. Doing a slight curved motion can make it less stressful for your shoulder. Perform as many repetitions as possible, then switch hands.
The Military 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.