Get a baseball
Get a throwing partner
Place your fingers on the two seams just before the point where the seams round off
Move your pointer finger over until it is touching your middle finger
Move your thumb until it is below your pointer finger. It will feel like you're only gripping one side of the ball but this is essential for good rotation
Apply pressure to your middle finger and your thumb. Imagine that you're trying to cut through the ball with your middle finger
As you pitch, you want to keep your fingers to the side, which is different from a fastball where you want to keep your fingers on top of the ball
This is an example of a fastball. Notice that with a fastball, my fingers are on top of the ball. With a curveball, your fingers are on the side.
Before you release the ball, your thumb should be facing your face and your wrist is slightly cocked in
This is what a fastball looks like
As you're releasing the ball, turn your wrist and your fingers forward like you're turning a doorknob.
Your follow through is the most important part. If your follow through is bad, you break will also be bad. When you release the ball, you need to try and touch the ground
Throwing a curveball