How do I do a tweeny shot without getting hurt or missing the shot?

First, start out slow and easy. Start at the net, and toss a ball up behind you (the ball should land at around the service line). Make sure when you first do it, you toss the ball slow and relatively low (3-4 ft over your head in a lob fashion). After tossing it back, turn around and run towards the ball. Once you get to it the ball should be coming down. When it's coming down pretend you have to run past the ball. Get directly over the falling ball and let the ball fall and PUSH your racket between your legs and hit it. DO NOT snap the racket down, that is how your hurt yourself (especially if you are a guy). Push the ball through your legs and finish with the racket through your legs. Once you can start getting it easily like that, start making the lobs harder and deeper. And remember the higher and slower a ball is the harder it is to do a tweener. Because the ball is so hard to time. So make sure you only use this shot when you absolutely need it!