For an infielder glove, I prefer to line up the tip of the pinky finger with the tip of the thumb and put a ball closer to the palm so that the pocket is created there instead of in the web. Having a deep pocket in the web didn't work for me as an infielder because it can make it difficult to get the ball out of there quickly enough.
For an outfielder, I just folded it where it felt the most natural with my hand in it and placed a ball in the deepest part of the webbing. The goal is to create a pocket that locks the ball in there. My best outfield gloves were one's where I could put a ball in the pocket, hold it face down (without my hand in it), and the ball stays in there on its own.
For both, rub in some oil, tie it up tight, and apply constant pressure. I used to put the glove between my mattress and box spring. It's not as much about purely flattening the glove as it is to get the glove to permanently wrap around the ball.
Have fun.