For cable one arm lateral raise you want to stand just behind the pulley and have your feet in a good stance. You're going to stand up, keep your core tight, and make sure that the pulley has tension on it at the bottom. Then, with a slightly bent elbow, you're going to come up just parallel and back down. Keep this motion as smooth and controlled as possible. You want the weight to come straight out and straight back down just right in front of you.