The arms have a long taper from full 5/4" thickness at the rear down to about 1/2" at the front.  I made a sled to hold the arm in place and elevate it at its front edge.  I did not have a planer at the time, so I spend a long time sending with many passes through the drum sander.

With some trial and error using some scrap, I worked out the angle for the arm support and where it would land on the underside of the arm.  After cutting the bevel at the top of the supports, I laid out the mortise locations and got them cut.  The arm and support will be glued together.

With the mortises done for both arms, I did the final shaping shaping on the arms and supports.

The supports received a 1/8" radius round-over.

The arms received 1/8" radius round-over as well, except on the long, inner edge, which was 3/8" radius.

Test fitting the arm and support, then the glue-up.

There's just some final fitting of the seat frame, and the chair will be complete.