I like to use a yard stick that is about an inch wide. Just wide enough for the ball to barely fit on it. I then putt it off the yard stick into a hole about 6 inches to a foot from the end of the stick and I make sure the ball has to travel for at least three feet on the yard stick. This lets me know I’m stroking the ball on my line. If you can putt it straight 3 feet off of a stick you can make any short putt. And it builds confidence in my stroke seeing the ball consistently fall off the end of the stick.