Curve ball is all in the wrist position. Really all are, though.
Hold your hand up like ‘taking an oath.’ Palm facing forward. That’s the fastball wrist position.
Now, turn your wrist toward you. Facing you, like you’re going to slap yourself. That’s the curve ball position. Hold a ball in that hand so your two fingers are on top of the ball and their tips are pointing directly at you.
Tight grip. No space between your hand and the ball. Throwing motion has that wrist position from hand break to release. No wrist snapping!!! Set the wrist position in the glove before hand break.
Now, normal throwing motion, except rotated wrist. Throw on fastball plane. Don’t ‘cast’ the ball. Don’t let your thumb ‘flip’ up on release, like you’re trying to spin the ball. Spin takes care of itself. Pull down with arm at release.
The idea is to make it look like a fastball, (on fastball plane) and suddenly drop or move to the side or a bit of both. Don’t fall in love with the specifics of 12 to 6. Let your curve be YOURS. Make it consistent, keep it on the fastball plane. Then work on ‘tunneling’, that is make all your pitches go thru the same small tunnel for the first 4 feet after release. If you get that down and that’s all you get down, it won’t matter which direction your curve breaks, they won’t be able to hit it anyway!!
BTW - rotating your wrist outward so the back of your hand is facing your head and palm is away - that’s the circle change and screwball wrist position. If you can hold your palm directly away from you without bending your wrist, you can throw a mean circle change and screw ball! If not, all is not lost. Keep practicing!!
Also - keep your normal arm slot. Very important!