Develop a Routine
In general you should treat putts much like you do full shots, in a sense that you do a pre-routine in terms of pre-putt routine or pre-shot routine that is the same from shot to shot or putt to putt. The one exception; however, is when you are doing rehearsal strokes aside the ball right before you are about to make your putting stroke you are trying to get the feel for the distance. You are trying to get a feel for how hard you want to pace the putter through the ball.
Get in the Goldilocks Zone
In this case you want to think about Goldilocks. Make a stroke and say, "That was too soft the ball would not have gotten to the hole." Make another stroke and say, "That was too hard the ball would have gone too far past the hole." Then another stroke and that was just right, that would have been exactly the right speed.
Once you get the right speed of the stroke you step right into the ball, make your stroke and the ball will go right in the hole. The only difference is that it may not always take you three strokes. Maybe the first rehearsal stroke you make is exactly the right one, if that is the case put the putter behind the ball and go directly into your stroke. Don’t try to make the other two strokes because they might not come off nearly as well as your first one did and then you will not have the right speed.