I teach a beginners curveball grip to younger pitchers who are learning a curveball for the first time because I feel that it's the easiest way to correctly learn proper spin. (But I also think this is a great grip for more advanced pitchers to use in a practice setting if you're having trouble with your breaking ball.)
Here's how it works: Grip a baseball leaving the index finger off – like you were pointing at something. (Your index finger will be used to aim the baseball at your target.)
Next, place your middle finger along the bottom seam of the baseball and place your thumb on the back seam (as shown in the middle picture above). When this pitch is thrown, your thumb should rotate upward, and your middle finger should snap downward while your index finger points in the direction of your target. This, of course, is the reason this pitch is great for beginners: the ball goes where your index finger points. The beginners curveball helps to align your hand and ball to the target.
Note: Because hitters at the college and professional levels may be able to pick up on the "raised" finger during this pitch's delivery, a beginners curve shouldn't be used past high school ball.
Resource from thecompletepitcher.com/pitching_grips.htm#q3