Take a marker and draw a straight line on the golf ball. Find a 10- to 15-foot straight putt on a practice green and aim the line of the golf ball toward the hole. Use that line to square the putter head to the hole and hit balls trying to hole them. The line on the golf ball will show whether your putter is square at impact. If the line on the golf ball wobbles to the right then the putter is open and if the line wobbles to the left then the putter is closed. Continue to hit balls until the line rolls straight toward the hole consistently.
Tie an 8-foot piece of string to the end of two tees and find a straight putt on a practice green that is eight feet long. Place one tee into the ground directly behind the hole and place the other tee into the ground so that the string is taut and it crosses over the golf hole. The string should be a few inches off the ground so that a golf ball and putter can fit under it. Place a golf ball directly under the string and practice hitting putts so that the ball stays aligned with the string all the way to the hole.
With the permission of the golf course, find a 10-foot straight putt and mark it with a chalk line. Place a golf ball on the chalk line and practice hitting putts that roll directly down the chalk line and into the golf hole.