This story is for you if...
• You miss a lot of putts you think are easy.
• When you miss, you don't know which part of your stroke is to blame.
Take those pennies you've been saving for a rainy day and fill a coin wrapper, then take the roll to the practice putting green, drop it about three feet from the hole on a flat section of the green and try to putt it into the hole. This is one of the all-time great homemade training aids. Since the roll is about the same length as your putterface, it can give you clues about your stroke that normally go undetected when you're practicing with balls, which only interact with the putterface at a single point. With the roll, you get contact across the entire length of the putterface, so any miscues in your stroke are immediately made apparent by the errant movement of the roll.
If the roll spins to the left, the toe of your putter is catching it first, which means you're closing the putterface through impact.
If the roll spins to the right, the heel of your putter is catching it first, which means you're opening the putterface through impact.
If the roll rolls (instead of spinning) but misses right, your stroke is too inside out. Your typical miss is a push.
If the roll rolls but misses to the left, you're cutting across the ball. Your typical miss is a pull.
Keep putting your pennies until you can consistently get the roll into the cup. When you do, you'll know that your path is correct and your putterface is square through impact.