Hitters never look worse than when they swing at a curve after guessing a fastball.
To throw the curve, grip the ball with your middle and index fingers across the seams at their widest part. Hold it farther back in your hand than the fastball, but don't let it touch your palm or you won't get enough spin. Your object here is to get more of your finger surface in contact with the ball. Curl your ring and little fingers into your palm. As you bring your arm forward in your motion, your wrist should be cocked and rotating inward.