Hold the ball so that the part facing away from you has the two seams going sideways, connected by a turn. This is the same orientation used for a four-seam fastball.
Hold the ball with an overhand grip.
Place your middle finger down the middle of the ball so that it's in the middle of the top seam. This will be the only finger that will be wrapped around the ball.
Snap your wrist when you throw the ball to generate backspin. The looser you hold the ball, the more velocity the pitch will have. However, holding the ball more tightly will generate more spin, which can cause the ball to have more late movement.