Over the years I’ve found that swinging the putter straight back and then down the line with the face kept square to the target through impact (call it a “linear” stroke) is the easiest way to start putts on the desired line. An arcing stroke can work but it takes a deft touch and better timing than Bob Hope.
As you start the club down keep your left arm straight so that your swing bottoms out under the ball. Let the club “kiss” the grass. No need to carve a huge divot. Don’t “hit” the ball. Accelerate smoothly through impact. Your goal: Reach max speed about four inches ahead of the ball.
If you swing down hard enough the force of impact—coupled with the eventual squaring of the face—will pop the ball up and out. Don’t even think about finishing your swing—leave the club buried in the sand.