Hold the dumbbells with palms facing each other and just wider than your shoulders. Feet apart, with slightly bent knees, tense your core pulling your ribcage down to your belly button, but still keeping your body upright. This bit is really important if you want to isolate the shoulders.
Push the dumbbells directly above the shoulders, using only the muscles in your arms and shoulders. At the end position your arms should be straight (make sure the dumbbells remain the same width apart) and knuckles facing the ceiling.
Once you max out, if you want to cheat a few reps then bend your knees a bit more with the dumbbells in the bottom position of the rep and then as you push them up in an explosive action, straighten your legs at the same time.