Here is a good video of Justin Henin.

Key Points

Use of non racket hand - your left hand - to start the backswing and so turn the shoulders.

She takes the racket back in a "loop" movement, you do not yet.

But second key point, look how at the end of the backswing the racket head is slightly lower than the wrist, and the butt of the racket points down the court.

Then look at the "windsreen wiper· type movement as the racket head goes up and acroos her body ending up outside the line of her shoulder.She finishes quite low for more top spin - you cannot tell from this video where her opponent was and so what Justin was doing with the shot.

Note as well when she hits the ball her weight comes from her back foot to her front foot, and on contact she is semi open.

Have fun