They continually change what they are trying to do, never knowing what is right or wrong. When I work with a golfer, I measure what he is doing and then we revert to the laws of physics - what makes the ball start on a straight line or move in the direction you want. I measure what he is doing that does not conform to fundamentals and we go from there. You can practice your whole life on something that does not work, and it still will not work. First, you have to aim the putter where you want the ball to go.
Most golfers aim poorly, so before they start they're fighting a loosing battle.
You can never develop a good stroke if you aim wrong because then, good strokes will make you miss. Next is the path of the putter. The direction the putter is moving in during the stroke should be along the line you want the ball to start on. The third perimeter is the face angle. It must be square, be perpendicular, to that line you're moving along.