The tapered center section of the newel is made up of four frame-and-panel assemblies. I transferred the dimensions and angles for the stiles and rails from my full-scale drawing, cut them to width and length from 3/4-inch poplar, and joined them together with pocket screws and glue.
I then cut a 3/8-inch rabbet around the inside of each frame with a router to accommodate the plywood panels, which I would make from readily available 3/4-inch birch plywood.
To safely cut the tapered 3/4-inch plywood panels, I made a plywood sled for the table saw and used the drop from the first angled cut as a spacer for cutting the remaining edges.
Instead of squaring the corners of my rabbet in the back of the frame with a chisel, I clipped the corners of each plywood panel with the miter saw.
I applied yellow glue to the rabbets, dropped the panels in place, and secured them with headless pins while the glue dried.
In preparation for assembly, I ripped 3/4 inch from the 1-7/8-inch-wide stiles of two of the frames. I also cut a 15-degree angle on the bottom rail of each frame, using the table-saw miter gauge and the wedge-shaped spacer to hold the frame square to the blade.
I glued the four frames together, pinning them to each other with my 23-gauge headless nailer to hold the assembly in place while I set up my clamps.
Setting the two wider frames slightly proud of the two narrower frames made it easier to flush-trim the joints with a router after the glue dried; this resulted in better-looking joints. I cleaned up the 26-inch-high assembly with a random-orbit sander and 120-grit paper.
While the tapered glue-up was drying, I made the four rectangular panels for the 9-inch-high upper assembly in the same way.