Sit or stand under the bar with it set at shoulder height, and unrack it by rotating your wrists to release the hooks. Press it straight up along the machine's fixed vertical track until your arms lock out overhead, then lower it back to shoulder height under control. Because the path is fixed, you don't need to actively guide the bar forward and back the way you would with a free barbell overhead press, which takes some of the coordination demand off your shoulders. Keep your ribs down and core braced rather than arching your lower back to help the bar past your face -- the fixed path won't correct that for you the way stepping back from a free bar would.