The Curveball is generally a more advanced pitch to learn and throw because of the unique mechanics involved. Many coaches feel that younger players (below the age of 15) should not practice throwing the Curveball with real baseballs because it might place too much stress on a young arm. Although it is likely much safer to throw curveballs with a Blitzball because of its light weight, younger players should still ask parents' for their approval before trying to learn it (you might want to learn a hard Knuckleball instead to get a similar dropping action without having to throw actual Curveballs). The Curveball is unusual because it is the only pitch that involves "topspin" (think of it as the opposite of a fastball's backspin).
In order to throw a proper Curveball, it is necessary to get "on top" of the ball and then spin it downwards. The grip is pretty simple: just put your index and middle fingers together and then place the middle finger alongside the Blitzball seam. You should get a good, tight grip on the seam with your middle finger so that you can really get some leverage on it.
When you throw the Curveball, you want to cock your wrist at a slight angle while keeping your middle finger high up on top of the ball so that you can roll your wrist downwards when you release the ball (almost like pulling down the string on a window shade). As a result, the Curveball should spin forward as it approaches the batter, giving the Blitzball a great downward movement. In terms of delivery, some people like to come almost straight over the top to get that great "12-to-6" / Barry-Zito-style drop (drops almost straight down) on their curveball, but this is difficult to do. Most people get better results with something closer to a three-quarters delivery for a "1-to-7" break (a pitch that drops and curves to the side). Try experimenting with different release points and arm slots to find the pitch that works most effectively for you.