The Chalk Line
Why this drill
A snapped chalk line from ball to hole is the most honest putting mirror ever invented: the ball either rolls down the blue line or it doesn't, and the eye learns what a truly straight start looks like — which is rarely what it believed.
Setup
A carpenter's chalk line snapped on a straight six-foot putt (find the flat with a ball roll first), or an alignment stick laid beside the line as a poor man's version. Five balls.
The drill
Roll putts one at a time straight down the line, watching the ball against the chalk for the first two feet — that's where the truth is. Count balls that hold the line into the cup out of ten. Then stand back and putt five more focusing only on the face at address: most players discover their "square" was a degree open, and the chalk had been correcting their aim by feel for years. Ten minutes on the line resets the eye for weeks.
One thought to take with you
Start it on the chalk — the hole is the line's problem.