Locate a tennis ball to use. The newer the ball, the easier it will be to throw a curveball. Tennis balls that are worn down have deep seams and are less likely to break.
Identify the spot on the tennis ball that makes a U-shape. Rotate the ball so that the U-shape appears upside down.
Place your index finger and your middle finger together so that there is no gap between them.
Place the fingers from Step 3 on the right seam of the upside-down U-shape.
Throw the curveball by rotating your wrist forward as you release the ball. Tennis balls are lighter than a normal baseball, so you need a very exaggerated the wrist rotation, harder than you would when throwing a regular baseball.