Seated dumbbell shoulder press is a great exercise to increase the strength in your shoulders. With dumbbells in both your hands, you are making balance on your body. While seated, you will not allow any other muscles to assist your shoulder muscle.
First of all, you are going to need a bench with back support. Sit down on it, so that your torso and your legs are making 90 degrees angle. Get dumbbells in both of your hands and put it on a position like on picture 1, so that your upper and lower hand make 90 degrees angle too. That will be your starting position.
As long as you are ready, start pushing the dumbbells upward over your head, until your elbows are fully locked out. Once your elbows are locked out, make a short pause like on picture 2.
After the pause is done, start lowering the dumbbells to its starting position. Make sure you are not lowering the dumbbells too much, only to their starting position. Make this exercise in 3-4 series, with about 12-15 reps per each of the series. Do not overweight, because you do not want your shoulders to get hurt. It is on you to pick your dumbbells weight.