Jennifer Douglas, Senior Pro player and teaching pro, demonstrates a simple way to practice the fundamentals of an overhead by yourself. Straddle the center line, step back with your dominant foot so that you are standing parallel to the centerline and your non-dominant hand is in front of you pointing up at the ball. When you drop your dominant leg, your dominant hand with the paddle should go up by your ear. When you are ready and set, toss the ball high enough so you have time to react and hit it at a high point. 

The beauty of this drill is you will get instant feedback from the centerline on the other side of the net. If you hit it straight, it’ll be right along the centerline, but if you are late, it could be to the right, or if you’re really fast and pull through and don't rotate your hips, It will go to the left (if you are a righty). Enjoy!