Place ball on green directly below the loop. When trying to achieve the correct head position the ball needs to be in the middle of the loop. As in diagram 1.
Diagram 1
When making a putt, ensure that the ball remains central.
Not as in diagrams 2 & 3
diagram 2 diagram 3
Body and head movement to the left will cause the ball to move to the left of the loop, as in diagram 2. The same applies to diagram 3 but to the right.
diagram 2 
diagram 3