An easy way to make sure that you are making square contact in putting is to draw a straight line around the circumference of a ball with a fine marker. I do this by carefully placing a rubber band around the ball so that it follows the circumference then draw the line following the edge of the rubber band.
Place the ball on a level putting surface with the line aligned with the direction you want to hit the ball. Make your stroke.
If the line wobbles or oscillates you have not struck the ball with your putter face square to the line and thus not made square contact. Practice this constantly till the line rotates smoothly without diversion for the first couple of feet of the putt.
You will know then that you have started the ball along the chosen line and made square contact.
If you are having problems making square contact it is time you had a look at your putting stroke. See the “putting stroke“ tip in this category for some ideas on how to improve this aspect of your game.
Square contact is very important. It means that you have sent the ball off along the line you have chosen. Your line may not be correct but you will never know until you are regularly making square contact in putting. Then you can start working on your line.