You can use either a barbell or a dumbbell.
Starting position – Sit up using either a chair type bench or a flat bench. Grip the barbell about shoulder width apart and hold the weight over your head.
Movement – Lower the weight down behind your head slowly until your arms are bent as far as they will go. You can lean your head forward to allow the weight to get back there. Then push it back up over your head slowly and under control.
Note – Try to keep your elbows in to get the full effect.