The most consistent way to swing a putter exactly the same, time after time, is to replicate the swing of a pendulum.

Try The One-Arm Putting Drill To Help Your Putting Stroke, Senior Tip
This type of swing is one of perfect rhythm and of equal swing length, backwards and forwards. Swinging in this way promotes both accuracy, as the putter always swings on the same line, and control of distance, as the length of the swing determines the length of the putt, which is entirely predictable.

To swing in this fashion requires the use of the bigger muscles of the shoulders while everything else remains still – the hands, arms, legs, hips and head. Unfortunately, most golfers do not use a pendulum-like swing. Instead, many golfers recruit the use of the smaller muscles such as the wrists which can produce a twitchy, flicking action where accuracy and distance control are unpredictable. If you are one of these golfers and wish to improve your putting stroke use the one arm putting drill to develop your pendulum putting action.

– Take some golf balls and stand three feet away from a hole.
– Before hitting any balls, take some practice swings, but with a perfect set up. Make sure that your eyes are directly over the ball, you have good posture, a solid base with the feet just over shoulder width apart and that the arms and putter are positioned in a 'Y' shape, hanging loosely from the shoulders.
– Once that you have checked these points, take the front hand off the golf club (the left hand for right handed golfers) and put it behind your back so that you are only taking practice swings with one hand.
– Focus on the rhythm of the swing of the putter head and gently move the putter backwards and forwards, letting the shoulders swing the putter using its own weight to power the action.
– Introduce the golf ball to this one handed motion and let the ball get in the way of the putter head rather than the putter head hitting the ball.
– Once you are confident, try this exercise from different distances and alternate putts with one hand and two hands. When putting two hands back on the club, make sure to only hold the club very lightly with that front hand so that it does not dominate the stroke again.

This drill works well because when one hand is taken away, the heavy weight of the putter makes it difficult to jerk or push the club through the impact area, ensuring that the putter swings naturally rather than forcefully.

Try it, develop a pendulum motion and hit more consistent putts – simple!