As mentioned earlier, the armrest will fit into a 1/8" deep recess on the top of the arm.  I made this router jig (from scraps) to use to cut the recess.  It will give me a perfectly flat bottom recess that's perpendicular to the sides of the arm.

The top of the jig provides a stable platform for routing the top edge of the arm and defines the boundaries for the recess for a pattern bit to follow.   The length of the opening is the exact length of the armrest (14").  The width of the opening is just wide enough so that the 3/8" diameter pattern bit will cut across the full width of the arm in the corners of the opening.

The underside of the jig has two lengths of plywood that center the arm in the opening.   A clamp at each end pinches the arm and keeps it in place while routing.

It's a little blurry, but the shows the recess in the captive arm after routing.

The finished recess.

Next is cutting the mortises for the floating tenons for all the joinery.