Using an alignment stick and a couple of tees, locate a relatively straight putt and use the alignment stick to represent your start line and make a ball gate about 10 inches away from where you will be hitting your putts.
This station will give you quick and definitive feedback on how your putter face is orientated through impact. You’ll know you’re leaving the face open if it leaks to the right, and the opposite is true if you’re hitting the tee on the left.
Success in this drill does not depend upon whether or not the ball goes in the hole; it’s a matter of getting the ball started on the intended line, which will result in more made putts assuming good reads on the course.