Putt to a tee in the ground with your eyes CLOSED. the aim is to feel where the ball ended up - just that.
Concentrate 100% on where you think the ball has finished, then look up and check. Make no attmept whatsoever to change the swing, address pos or anything else. just feel where you think the ball is going, see if you can improve where the ball finishes compared to where you thought it went.
You can obviously play golf. the aim of this drill is to get back to the basics of touch and feel, with no pressure of the hole to concern you. It is NOT generally for use on the course unless you are really struggling.
Half an hour of this has never failed me, and I am generally regarded as a really good putter. When you can play the game well but the putts are not quite dropping, its normally the feel you need to regain. Sometimes you feel where things are not quite right and naturally adjust - last time - April - I could feel that impact was generally just a little towards the toe away from plumb centre.
Just a 1/4 inch adjustment was all it took for me to feel every putt out of the middle again, with immediate improvement in results on the course.
Main source of this was "Inner Game of Golf" - a must for all of us who can only practise 2 or 3 hours a week at most.