In order to be fully stable overhead, you need to learn what that position feels like. This can be a difficult task when you are training by yourself. The drill in this video shows you how to find that strong, reliable overhead position on your own.
Put your hands straight out front. Then, pull your shoulders back and pinch your shoulder blades together. Then, rotate your arms to an overhead position, leading with your pinkies. From there, feel the position your back and arms are in!