I use a great aid when helping my students to understand and track on the proper arc. This aid is called the Putting Arc. It is a wooden board (it also comes in a plastic model) and one side has a slight arc to it. It also has slits in the board that are square to the arc.
I have painted these slits with white pant. When you place the heel of the putter on the board you can practice your stroke where the heel of the putter stays on the board and the face of the putter lines up with the slits on the board.
It helps to once again groove this arcing motion. You would do it first without a ball and then with a ball but not on the putting green but on carpet. You can aim at a target about 10 feet away. Here is the important part. Put a board or large book at an angle in line with your target. When you hit the ball, the board will deflect the ball away from your target.
The reason you want to do this is you do not want to judge your putt from the stand point if it hit your target. You are working on your stroke not trying to make the putt go to a target or hole.
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