The two most important things in putter fitting is to find a putter the player can AIM and also one that they like the way it looks and feels.
That said–putting is very independent and personal. Being able to aim the putter is key, using a line on the ball or the side pole trademark can help with this a lot.
The position and alignment of the body is not as crucial, but a preference would be to have the shoulders, forearms and eyes as close to parallel to the start line as possible. A lot of folks get the shoulders and forearms open, this causes an outside to in stroke. Other’s tend to cock their head one way or another causing their eye line to be off, you’ll tend to swing the club in the direction the eye line is looking. The lower body, hips and feet are really not much of a factor since all the motion is with the shoulders and arms.
Get set-up in your normal putting stance/posture and have someone lay an alignment rod across your shoulders, then your forearms and then under your eye line and see where they are pointing. As I mentioned using the line on the ball and hopefully the line on your putter to help it get aimed correctly. If there are issues a lot of practice in front of a mirror on the set-up will help.