Putting is the most precise part of the golf game. The margin for error is minuscule. So then the target you choose to aim at must be small and precise. I find that the best way to do this is to use a golf ball placed 4-6 inches in front of the hole. The ball is placed so that it is directly in the way of a made putt. If you can hit this target you can hit the much bigger one beyond it.
To practice this drill, start by setting up to a reasonably straight putt and put a ball in the way. See how many times in a row you can hit the ball with a putt. When you feel confident with hitting your target remove the ball and set up again this time visualizing the ball in the way. You will notice that you will be more focused and accurate with your putting.
If you are having trouble hitting the target during the drill you may be thinking about technique and not letting your natural athleticism work for you. Forget about everything but hitting the target, just react to the target and see if this helps you. If not move closer until you can hit 8 out of 10 and keep moving back until you are hitting at least 60 percent of your targets from 10 feet.
Hit the target and the putts will start to drop. Be athletic and focus on the target, not the stroke. I guarantee that this game of inches will become much easier with this simple drill.