Once you have mastered an efficient putting stroke, you need to learn how to read a green.
Spend time assessing the slope of the green and judging how much the slope will affect the roll of the ball and the pace of your putt so you can decide on the speed of your stroke and the length of your backswing.
Standing behind the ball and looking at the hole will give you a good overall view of the contours between the ball and cup. Try to squat down close to ground level (no need to lie down!) as this will help you to see if the green is level or if the left or right side of the hole is higher or lower to ground level. If it’s level, it’s a straight putt, but if higher or lower either side of the hole, then you need to make a decision as to where to aim your putter.
Once you decide to take aim, believe from that point that you are making a straight putt and allow the natural slope of the green to feed the ball into the cup.