Using a machine for shoulder presses will be safer if you exercise without a spotter. The movement is more controlled and therefore does not work the stabilizer muscles as hard as dumbbell shoulder press.
Begin by sitting down on the shoulder press machine and grasping the handles. Make sure that the seat height is appropriate; the handles should be just above your shoulder height. Now, push the handles up until your arms are fully extended. Hold the contraction at the top position for a second. Then, return the handles to the starting position and repeat until you have completed your set.
Keep your back flat against the back pad; do not arch your back at all.
On many machines you can use palms facing forward grip or a neutral grip (palms facing each other). You can use either grip or vary your grips from time to time.