Start off by placing the putter against the base board of the wall. Then setup up to the putter, focus on having your arms hanging by your shoulders and then place the crown of your head against the wall. Once your setup, let the putter swing on an arc touching the bottom wall board at impact or at the apex of the arc. Alternatively start by taking it away from the wall, then come into the wall, then back out.  Having the head against the wall should also give a create a good feeling of how the back should be angled for the putting stroke

This drill will help create a still head during the stroke, allow the eyes to be over the ball and allow the arms to hang correctly