One of the stepping stones of an excellent putting is a correct stance, which will be easier to achieve with the help of a mirror.
Two crucial aspects to check are related to the eye vertical line and to the alignment of the shaft. To easily define the eye position versus the line of stroke take your usual stance and then rip off your left hand from the grip. Then seize in your hand the ball you‘ll have previously placed in your pocket without moving.
Leave the ball fall down from your left eye (Picture 1) and see where it fails. You will only have tree different alternatives: beyond the target line, on the target line or between the ball and your feet, within the target line.
Please also note that contrary to what is commonly taken as correct, it’s not always true that keeping the eyeline above the target line is the ideal position. For many golfers is preferable keeping a position slightly within the target line – within the ball and the feet – in order to improve their shot.