Being able to control the face of the putter through impact is priority #1. This enables you to hit the sweet spot of your putter consistently with a consistent face position that allows the ball to start on and stay on the start line of any putt.
Use rubber bands to highlight the sweet spot of your putter to enhance your ability to hit the sweet spot every time. If you’re off and hit the rubber bands, you’ll feel it, and the ball will not stay on line.
Use a yard stick to understand many things, face square at set up, intended line identification, and most importantly, to work on your ability to initially keep every putt on its intended line.