Look Down Vertically On The Ball
Your eyes are positioned directly above the putt's starting line.
I don't think the particulars of how you set up to putts matter much, as long as you feel totally comfortable over the ball, which to me mostly requires being solidly balanced and physically stable. That's because awkwardness or discomfort in the setup almost invariably inhibits the smoothness of the putting stroke.
You must look down vertically, however. This means that your eyes are positioned directly above the putt's starting line rather than inside or outside of it.
If your eyes are beyond the line, you'll tend to pull putts left; inside the line, and you'll likely push putts right. To check this, first assume your usual head position over the ball, then drop another ball straight down from the bridge of your nose. The dropped ball should land on your putting line.
KEEP YOUR LEFT SHOULDER STILL AND LOW
A focal point for me in putting has always been my left shoulder as a contributor to lifting or twisting the putterface.
My wife, Barbara, would often tell me after a poor putting round: "You're lifting your head again!" Actually, a good part of the time I wasn't.
What sometimes happened was that I unconsciously elevated my left shoulder on the through-stroke, which caused my head to rise.
If you're a habitual head-mover, as so many higher-handicappers are, as you stroke through the ball, keep your left shoulder low and still. Not letting it come up even the tiniest bit until the putt is on its way will encourage your hands and arms to move through correctly, and will teach you to follow the ball's roll by swiveling, rather than raising, your head.