Then, I placed the metal plate in the center of each post location and masked around the outer edge. Using a router with a 1/2” cutter, I set the depth of the cut to slightly more than the thickness of the plate.
This will ensure that once the plate is secured it will be slifhtly below the floor surface and make for a tight fit between the newel post and the floor.
With both hands on the router I free-handed the mortise in the floor.
Because the metal plates have a smaller radius on their corners I had to “over rout” each corner slightly so the plate would drop into the mortise and could be fastened with the four long screws.