Position
Stand with your feet just wider than shoulder-width apart. Bend your knees and hips, with your trunk between 45 and 75 degrees in relation to the floor. Maintain a natural arch in your back. Keep the weight over your ankles. Hold the dumbbells perpendicular to your body, your arms straight down, and your elbows pointed out and slightly bent. Pull and hold your shoulder blades tightly together throughout the exercise. Begin with your arms slightly out so there is some degree of gravitational pull on the dumbbells.

Technique
Contract your posterior deltoids. Bend your elbows and pull your arms apart and up until your upper arms are approximately parallel to the floor, forming about a 90-degree angle. Hold and continue to contract your posterior deltoids. Keep your arms relaxed, elbows pointing up, and scapulae retracted. Slowly lower your arms back to their original start position, keeping your scapulae retracted and tension on your posterior deltoids. Repeat.