A good key to use when learning this position is to point your front shoulder blade toward the ball. If the ball were to fall all the way, it would hit the player on the front shoulder blade.
The big difference in the unit turn compared with the forehand, of course, is that as you turn on the overhead, the arms start to rise upwards above the shoulders.
But the wind-up here is not identical to the serve. Unlike the serve where players usually drop the racket arm at least initially in the wind-up, the arm motion is directly upward. The overhead wind-up motion is actually more similar to the super-abbreviated serve wind-up of a player like Andy Roddick than it is to most serve wind-ups, probably including yours. So don't make another critical mistake of using a long, circular motion.
After the wind-up starts upwards, it continues until you are in a position resembling the trophy position on the serve. I say resembling because there isn't an exact match. Typically, the player's left arm will be pointed directly upward similar to the service toss but sometimes it's even higher. And it can be substantially lower when the time is short.
Also, the racket is often further along in the motion. In the classic trophy position, the racket tip points directly upward with the arm in an L shape at the elbow. On the overhead, however, the players have usually begun the racket drop, rotating the upper arm backwards, so the racket is pointing more sideways toward the sideline.
The amount of time you have on the overhead is another big difference with the serve. The timing of serve is the same with every ball, controlled by the toss height. Because players are hitting a moving ball on the overhead, the timing can be very quick or take up to several seconds. The exact position of the arms varies with this timing.
Sometimes when the time interval is very short, the left arm extends less than a serve. If the players have a large distance to cover, however, they may hold the racket longer in something closer to the classic serve trophy position as they move before beginning the racket drop when they reach to set up position.
Another difference compared to the serve is the distribution of your weight at the time the left arm extends. If you can get set, then the weight will be more balanced between the feet. This can be similar to the serve.
But if you are moving backwards, your weight will typically be more on the rear foot. This is what makes the jump overhead possible when the players are moving back and then launch themselves upward to the hit by pushing off the back foot.
Beyond these technical differences in the preparation, the biggest difference with the serve is that the player must move to the ball.
How a player moves his/her feet to position themselves on any shot is key to achieving great success in your game. But on the overhead, the footwork is especially important and can be the difference between having no overhead and a having a very good on.