This video explains the importance of marking your ball on the putting green. If someone else is also playing golf, and theirs hits yours (and it isn't marked), its a two stroke penalty. In order to mark the ball, set a coin directly behind the ball onto the green. Then when you replace your ball, set it directly in front of the coin. It is important to always put your ball in front of the coin so you don't accidentally gain or lose advantage. When putting your marker in place, make sure to note something nearby so you know which direction to face with your club head. Make sure to replace the marker each time you measure with the club head.