Attached are some photos showing where I have the extra bearing mounted. I just set the bearing on top of the metal tray shaft hole, then I replaced the tray so that the collar sits directly on top of the bearing. This results in very free and fluid movement of the tray which holds the lower baking stone. Don't forget to grease the bearing before replacing the tray.
The first photo shows the bearing sitting beside the metal tray shaft hole.
The second photo shows the bearing in place before replacing the metal tray for the lower stone.
The third photo shows the metal tray replaced, with the collar sitting atop the bearing.