Prior to working on the mechanics of any particular pitch, you must get yourself in top shape and build your arm strength. The best way to do this is to throw regularly in the offseason so your arm does not get overworked when you report to training camp. If you live in a cold-weather area, throw for at least 15 minutes two or three times per week inside the gym.
As you get more opportunity to throw, play long toss with a teammate. Instead of throwing from 60 or 70 feet, stretch the distance out to 100 feet. This will get your arm prepared for the rigors of pitching during the season.
Grip the baseball with the seams when you are preparing to throw an overhand curveball. Your fingers should be on the laces and your thumb should be on the bottom of the ball. Your fingers should be next to each other on the laces. If you were throwing a fastball, your fingers would be spread with a wide grip and your fingers would be across the laces.
As your begin your pitching motion, you will stride toward home plate with your left leg if you are right-handed. As your foot hits the ground, you will rotate your thumb as you release the ball. As the ball leaves your hand, your thumb is now on top of the ball.
The faster you can make this twisting motion, the more the ball will curve during the course of the pitch. If you are a right-handed pitcher, the curveball can be most effective when it is thrown to a right-handed batter. The spin you impart on that pitch will make the ball curve down and away from a right-handed hitter.
You can also throw it to a left-handed hitter, but it will break towards the hitter and unless you spot it perfectly, it will be a much more hittable pitch.
Keep the ball low when you are throwing a curveball. A curveball that is at waist-level or higher is much easier for most batters to hit. When the pitch is low in the strike zone, it is more difficult to hit the curveball with authority.