Hold the bowling ball with your two middle fingers in the finger holes and your thumb in the thumb hole. The farther you can set your finger holes and thumb hole apart, the easier it will be to put revs on the ball. Most bowlers now use a fingertip grip, in which the thumb is mostly inside the ball but only the fingertips are in the finger holes. Experiment with this to see that you can still hold and maintain control of the ball.
Swing your arm through your throwing motion, keeping your hand underneath the ball, with your wrist cupped.
Concentrate on getting your thumb free of the ball first, at release. Lift your fingers upward as they come out of the ball.
Move your hand up and rotated it in a counter-clockwise motion (if you are right handed) as you throw the ball. This will add sideways revolutions to the ball.