Take a dumbbell and sit on a flat bench. Place the dumbbell on your lap, with its handle vertical. Put your palms on the lower plates, with your thumbs curled around the bar. 
Push the dumbbell up with your leg and put it on your shoulder, turning it upside down while you lift it. Now your palms are facing up and pressing against the inside of the top plates.
Push the dumbbell up until your arms are almost straight.
slowly lower the dumbbell behind your head as far as possible. Keep your upper arms vertical and motionless and your elbows close to your head throughout the exercise.
Slowly extend your arms over your head.