Try this exercise to get a feel for the difference. Take your shoulders and squeeze then behind you as though you were trying to touch your elbows behind your back. Now bend over the ball as if you are preparing to putt.
Doing this prevents you from using your back to bend over. You now know how it feels when you bend from your hips instead of your back.
While in this position let your arms hang naturally at your side. Bend your elbows slightly. This should feel natural and is the best way to setup to putt.
The next thing is to position your golf ball in the optimum location. The position of the ball will affect the way it rolls on the green. I have found that the best position is when the ball is directly under my eyes. This allows me to be able to peer directly down the path that I want the ball to travel when it leaves the putter.
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Place your putter in the middle of your stance, front to back, and then place the ball directly in front of the putter. As you will see, the ball will be about 1 inch or so forward of your center position.
Your body and ball are now in the correct position to allow for a simple repeatable putting stroke.