do you know the standard curveball grip? (pointer and middle finger together against one seam, and the thumb on the opposite seam)
if so use that
BUT!! make sure that your thumb is bent at 90 degrees like hitchhikers thumb. This will cause it to partially be off the the ball put the part that is on should help push the ball for rotation.
also make sure you don't throw it side arm like a slider or a slurve. Throw it over the top.
*You should throw it like your fastball (armslot,...)
AND follow through on it. Don't make the the big mistake of stopping half way through after the ball is thrown. follow through like ur FB
hopefully this helped