With the putting mirror you can easily use the mirrored surface to check
that your eyes are over the ball, shoulders are lined up, the putter face is square as well as get feedback as to your body movement and putter path during the stroke.
HERE'S HOW TO USE IT
Step 1 - Precision Alignment to target line
The first step in using the mirror is to accurately align it to your target.
Step behind the mirror and aim the center black line exactly at the target.
The smaller your target, the more accurate your alignment will be.
Take your stance over the ball.
Don't be surprised if your eyes try to tell you your line is off. You may have been seeing incorrectly for years!
Step 2 - Eyes over the ball
Look down at the ball. Your eyes should be directly over the ball and the centerline. This could take some adjustment in
Your posture
The position of your hands at address
Your distance from the ball.
Step 3 - Square putter face to target
Square your putter face to the target line.
Use the line directly behind the ball as your reference. You should be able to see the line directly in front of the putter blade.
Step 4 - Shoulders square to target line
Since the putting motion is a rocking of the arms and shoulders, the shoulders need to be lined up to the target
Look at the bottom of the mirror nearest your feet. You should be able to see the tops of both shoulders.
If you see only your back shoulder, you are open to the target line. If you see only your front shoulder, you are closed to the line
Watch your shoulders as you make a stroke. Are they moving back and forth down the line.
Step 5 - Learn from your reflection
Watch your shoulders and head position during the stroke. If the reflection moves in the mirror then your body is moving while watching your shoulders will reveal the plane of your your shoulders rotation | 
