I spent years reviewing the fitting process for putters and love the The Process to Putter Fitting from SeeMore Golf. The Rifle Scope Technology (RST)in their putters train the golfer to have a consistently square setup and posture on every putt.
The Fitting Process: 1) Static measurement for length. 2) Establish posture and tension free setup. 3) Change length of putter so eyes fall over the heel of the putter. 4) Check Alignment for consistency. 5) Adjust lie so sole of putter is completely flat on the ground. 6) Check loft for adjustment.