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
Sit down on a military press bench or adjust an adjustable angle bench, setting the back to 90 degrees.
Load a barbell with the appropriate weight. Ideally, you can un-rack the weight from a weight rack or have a training partner hand you the bar.
Hold the bar with an overhand grip (palms facing forward) with your hands just beyond shoulder width. Your arms will now be almost fully extended above your head with a slight bend in your elbows. This is the starting position.
Slowly begin lowering the bar down behind your neck as far as comfortably possible.
Once your upper arms are slightly past parallel to the floor, pause, and then slowly raise the barbell back up without locking the elbows out at the top of the movement.