It can be awfully frustrating when you read a putt perfectly, give it a confident rap and then see it roll nowhere near the line you picked.
This is a case of the putterface not being square to your intended line when you hit the ball. Face alignment is, by miles, the biggest influencer of direction. The trick is to set the face accurately, and then return it at impact to the same position.
First, square the face behind the ball and take your grip. Look at the position of the back of your left hand (for righties). Then, just focus on that hand: If it comes back at impact looking the same way it did at address, the face is square. This will happen if the hands work as a team, without one overpowering the other.
During the stroke, one of two things often goes wrong. Some golfers flip the right wrist at impact, which over-accelerates the putterhead and closes the face. (Those putts miss to the left.) Others shove the left hand toward the target on the forward stroke, which over-accelerates the handle and leaves the face open. (Those putts miss right.)
Instead, match the acceleration of your hands and the putterhead. Do that, and you'll like what you see when you look up.
Grab a striped range ball and find a straight eight-foot putt. Using two tees, create a "gate" a little wider than the ball and about 18 inches toward the hole. Then set the ball down so the stripe is perfectly vertical, facing the cup, and stroke the putt. If the stripe stays steady during the roll—no wobbling—and the ball goes through the gate, your putterface was square to the hole at impact. That putt's going in.
Here's how to find out if your putter is open or closed at impact. Place two balls on a green side by side. Then square your putterface behind the balls and try to strike them at exactly the same time. If the outside ball rolls farther, the face was closed at impact; if the inside ball rolls farther, the face was open. When they go the same distance, the face is square.