Holding the Ball
Before you ever throw a ball with two fingers, you need to understand how to hold it. Your middle and ring finger should be in the two forward holes. If they don't fit all the way in, it's all right -- you are most likely holding a finger-tip ball, which is good for curvature and safety. The weight of the ball should rest in the palm of your hand, and your thumb should be extended up the back of the ball for stability and support. Your index and pinky fingers should be extended as well for lateral balance. The ball should remain in the palm of your hand until your throw has accelerated the ball enough so it does not fall straight down when you begin to twist your wrist. This point will normally be somewhere just beyond the point where your arm is straight down during the throw.
Knowing the physics behind bowling is the first and most important step to mastering any bowling style. You have to understand how and why a ball curves as it goes down the lane in order to make adjustments for it. The ball curves because as it comes off your hand, you rotate your hand from palm up to palm sideways, giving it a little bit of roll in addition to its forward momentum. Two-finger bowlers have the potential to make much greater turns, because the thumb is out of the way and not involved. By the same token, however, the thumb in traditional bowling adds stability and strength that you will lack when bowling with two fingers.
Knowing the physics, you can start to adjust your throw for the two-fingered style. Like marksmanship with a rifle, you only get better by throwing a lot of shots downrange. You can't make huge adjustments between every shot. For two-fingered bowlers, it is important to develop, over time, a style that gives you accurate control of both your spin and forward momentum, and a safe release that won't harm your fingers. Start off slow, as bowling with two fingers can be dangerous if done improperly. If your fingers are hurting, you are doing it wrong. Stop and seek the help of a coach. Alternatively, you might ask a bowling lane attendant: many are good bowlers themselves and can give you more pointers.
While any style can be successful in the long run, there is a reason the two-fingers-and-a-thumb style is preferred over the two-fingered style: the two-fingered style does not lend itself well to accuracy and repetition. Accuracy is hitting the "sweet spot," while repetition is hitting any spot from shot to shot. Therefore, you absolutely must practice for all spare scenarios. A bowler who knows how to hit his spares can bowl a 190 -- a really good score -- without bowling a single strike.