Great putters are consistent. What makes consistency is a great setup, and there are three keys to achieving this setup.
The first is ball position. Get the back of the ball in the middle of your stance so that the strike point is at the bottom of your arc. Second, your shaft in relation to your lead forearm. Ensure that the shaft is running up your lead forearm. The last key to a consistent setup is being athletic. Don’t get too rigid or mechanical in your stance.