The onehanded backhand is executed by pulling and lifting a locked hitting arm in an inside out path. The first position you should practice is the extreme inside out approach that all pros use. In this frame, the racket has dropped below the ball and notice how the butt of the racket is pointing straight out to the left. Notice as well how the racket face is directly facing the back fence and the racket is in close to the body. And finally, see how both Joachim, Federer and Guga's left hands are on the throat of the racket.

Most people don't get into this full inside out position and don't let the racket fall into the body like you see here. I think you'll be amazed at how much easier it is to hit the one hander with a full inside out approach to the ball.