Being a good putter means being accurate. To consistently roll the ball with precision, the putterhead has to move along the target line just prior to and after impact, and the putterface has to be square to the target line when it hits the ball. For an investment of only 50 cents, you can improve your ability to do both.
Roll 50 pennies in a wrapper, lay it on a practice green or carpet, and try rolling it with your putter as you would a ball. Because the penny-roll is cylindrical and much wider than a ball, you’ll immediately learn where your putterface is pointing.
If it’s open at impact, the heel will make contact first. If it’s closed, the toe will strike the penny-roll first. Either mishit will cause it to twist and bounce rather than roll. If the face meets the penny-roll flush, your face alignment at impact is perfect. And to make the cylinder roll straight down your intended line (it won’t roll far, because it’s heavy), you’ll need to swing the putterhead so it approaches on a path directly along the target line and continues in this direction until after impact.
Finally, because the penny-roll weighs more than a golf ball, you’ll be forced to keep your left wrist firm through impact and accelerate smoothly down the target line — two excellent fundamentals exhibited by all good putters.