This is how you hold the grip for a screw ball. You put your index finger to the left side of the first seem and your middle finger on the left side of the second seem.
When you are releasing the ball you want to focus on keeping your arm close to your body. The idea of this pitch when you are releasing it to is think about turning a door handle. It is also beneficial to take a step out a little bit more than usual.
The screw ball is a handy pitch to have in your arsenal. It keeps batters off the plate and you will always have them thinking.