A good curveball grip for beginner pitchers has the middle finger along the baseball's bottom seam and the thumb along the back seam. The ring finger and pinkie grip next to the middle finger in a natural position. The index finger is off the ball and points towards the target.
As the pitcher throws, the thumb rotates upwards as the middle finger snaps downwards, giving the baseball its top-to-bottom spin. Pointing the index finger in the direction of the target makes the throw easier to learn, but advanced hitters may use it to predict where the pitch is going. The pitcher places the index finger on the baseball after learning the pitch.