As this is an isolation movement, form is more important that weight.
Keep the delts under strain by using strict form.
Keep your elbows high throughout the movement.
Dont flare your hands up as you move the weight. This will bring the front delts into the exercise.
Instructions
Set the pulleys of a dual cable machine to the low setting and select the weight you wish to use. Stand facing away from the machine and grasp the right handle with your left hand.
Stand straight up with your arms in front of you (crossed over). The weight should be slightly off the stack. This is the starting position for the movement.
Keeping your elbow high, extend your left arms out in a semi-circle motion.
Once your hand get to around shoulder height, pause and slowly lower the weight back to the starting position.
Repeat for desired reps and then repeat with your right arm, grasping the left handle with your right hand.