To avoid injury, do not lower the bar much further than the top of you head when bringing the bar down behind you. This will keep your arms at a 90 degree angle to the floor and place less stress on your shoulder joints.
Keep your back straight throughout the movement. Dont let it arc too much when pressing the weight.
Use a lighter weight that you would on a regular barbell press.
Use slow and controlled movement, both when pressing and when lowering the weight.
Instructions
Load a barbell with the appropriate weight. Ideally, you can un-rack the weight from a squat rack or have a training partner hand you the bar.
Stand with your back straight and a slight bend in your knees. Your feet should be around shoulder width apart.
Hold the bar with an overhand grip (palms facing forward) with your hands just beyond shoulder width.
Push your the bar above your head until your arms are almost fully extended above your head with a slight bend in your elbows. This is the starting position.
Slowly begin lowering the bar to your upper chest - almost touching your collarbone.
Immediately begin pushing the bar back up to the starting position.
Once the bar has reached the height of the movement, slowly lower the bar down behind your neck as far as comfortably possible.
Pause, and then slowly raise the barbell back up without locking the elbows out at the top of the movement.