Click the link above to see the drill in motion
A quality putting stroke is one that you can repeat over and over again. Its's also one that puts a confident, aggressive (though appropriate) stroke on the ball. A perfect stroke will do all of those things while starting the ball in a rolling motion down your target line.
If you find yourself leaving them short or trying to steer or guide the ball into the hole rather than just putting a confident stroke on the ball and trusting your line, this drill is for you.
Grab that Bloody Mary or can of beer out of your cart (any bottle or cup will do), set it on the green, and practice your 6 and 8 footers by putting the the cup/bottle - not to a hole. It's a mental thing that causes a human response to swing more confidently and aggressively to a target that is above the ground rather than one that is below the geound, ie a golf hole. See if you start stroking it a bit more confidently and consistently with your new target.
Now try to visualize that out on the course! Good luck.