As amateurs, we tend to be too critical of our own putting because we have an unrealistic view of tour players. The percentage of putts made on tour from 3, 6, 9 & 12 feet are 90%, 58%, 36% & 20% respectively. Most amateurs are as good at putting as most tour players. The difference is simple. Tour players generally get up and down with one putt from inside 6 feet while amateurs take two putts. Tour players typically two putt from 20 to 50 feet while amateurs three putt.
Keeping in mind, 70% of all putts come from inside 12 feet, we should then focus on getting up and down in one putt from inside 6 feet and two putts from 20 to 50 feet. To improve in both areas, the idea is the same, roll the the ball to the hole at the proper speed.
Rolling the ball requires the upper portion of the putter face to strike the ball above the equator. To do this, pick a soft aiming point on the back of the ball such as a colored-in dimple. My back stroke is low and slow to promote an aggressive rise or launch angle on the forward stroke ensuring top spin on the ball. The overall stroke should be a 1:1.25 ratio, meaning the back stroke to forward stroke ratio. I do this by taking the putter face back to the big toe on my rear foot then accelerating through until the back of the putter head is just past the little toe on the front foot. This also promotes the ideal rise or launch angle. Most amateurs have too long of a back stroke causing them to have too steep of an angle and to decelerate resulting in an extending the forward stroke too far which decreases the rise angle or launch angle causing back spin on the ball.
Regardless of the length of the putt, I pick an intermediate spot between the ball and the target. This is called spot putting. The spot is generally no more than 6 inches in front of the ball. It is easier to aim at a spot 6 inches in front of the ball than a target 6 feet away. This also promotes rolling the ball instead of hitting the ball. When we aim at a target more than 6 feet away, the tendency is to feel the need to hit the ball in order to get it to the target.
Every putt is makable as long as the sped is correct so distance control is more important than accuracy. Work on proper speed by using your power arm only. Your power arm is your dominant arm and generally your rear arm. Combined with spot putting, practice from 3, 6, 9 & 12 feet. Then practice from outside 20 feet up to 50 feet.