I started installation by attaching plywood backing to the brick with masonry screws, using a plumb laser and shims to get everything in the same plane
I then installed the four pilasters and a quirk-and-bead around the marble slip, shown shimmed into place while the glue was setting.
The center bump-out is two separate pieces and sits a little proud of the pilasters; the bottom piece is wrapped with 1/4-inch MDF and hangs from the two pieces of plywood backing.
Coping was impossible for the detailed molding profiles, so I mitered inside corners. I started with the large cove molding at the top, then used gauge blocks cut to the width of the reveals to make sure the moldings below ran parallel.