When reading a putt's break, it's important to keep in mind that two major factors impact how much it breaks: speed and slope. When both of those ingredients are perfectly aligned, it's a winning combination. Getting there, however, isn't as easy as it sounds. Take my example here. The five balls bending to the left show the shape of my putt, a real right-to-left swinger that takes a nosedive in the last couple feet or so. The middle ball is the apex of my putt's break and, hence, where I'm aimed (notice how my body and putterblade are aligned toward that spot). Even though I want the ball to go in the hole, I've chosen the apex point as a target. (In my mind, all putts–even this one–are straight.) Keeping in mind the length of my putt, I putt toward the middle ball. From that point on, the ball slows down and takes the shape of the hill, breaking hard to the left.
As with any breaking putt, the last 1__ã3 or so of it will break the most as the ball loses steam and takes the shape of the green, so as you read the slope, keep the last few feet in mind.
There are two major factors affecting the break of a putt: the green's slope and the pace at which you hit the ball.