The bent over cable lateral raise is a good exercise for targeting your rear deltoids. The cable machine offers a consistent resistance than a a dumbbell bent over raise.
Steps
First attach a handle to the cable machine. Stand with your side facing to the cable machine. Bend your knees and your hips to get yourself into the bent over position. Don't bend your back. From the side you should look like a lighting bold symbol.
Grab the cable handle with the arm furthest from the cable machine. Your arm should be going across the front of your body. Raise your arm across your body and out to the side. Continue until your arm is level with the ground. Hold the top position for a second then lower your arm again and return the the starting position.