The third panel, with corner frame attached, is slipped over the second plywood panel.
The rail is glued.
The rail is pinned.
Because the 1/4-inch plywood panels are fairly flexible, the author is able to insert the fourth panel by pulling apart the two sides and springing it into place.
The panel is bedded in construction adhesive and pinned from the ends.
The newel is finished with a 12-inch-wide oak base, then capped and trimmed on site.