Ah, the curveball - that first hurdle every beginner bowler struggles with. When I first learned, I practiced for weeks with a tennis ball before I got it right.
Basically, I think you want to keep your stance exactly the same as you would throwing a straight ball; you just release it differently.
As you release, take your thumb out of the thumb hole first and control the spin with the two fingers that remain inside the ball. It might be easier to think of it like going from a palm-up bowling position to the kind of hand position you’d have if you were about to shake someone’s hand.
Obviously, this all happens in a split second, so it takes a lot of practice to get it right. Once you’re skilled at it, you might find it’s easier to just keep your thumbs out of the hole right from the start.
Also, before you practice throwing a curveball, make sure you’ve nailed the basics of throwing a straight ball. If you don’t know how to do that right, I found a great guide on https://aboutbowlingballs.com/ - so check that out. They also have some useful guides on bowling equipment like balls and bowling ball bags.
Anyway, good luck!
