Each one of the diagonals had to be marked to the length and angle that was subtended at the other end. They should have been all roughly the same but due to discrepancies in machining there are always slight differences.
The intersect with the mid rail was marked onto the diagonal. This angle was transferred to the table saw mitre gauge using a sliding bevel. The cut was made by sneaking up to get a good match. Then the piece was dry fitted into place. The edges of the diagonal was transferred onto the mid rail. The diagonal between the mid rail and top rail was lined up with the pencil lines.
This same process was then used to cut its upper angle.
The decision had also been made early in the design to use floating tenons to anchor the ends of the diagonals to the crossmembers. This required cutting 10mm (3/8") slots into each piece using a router. If you have a Festool Domino XL then use that instead. I don't so used the router method.
My mortises were cut 25mm (1") deep and the slot length is 100mm (4"). I marked the start and end of each mortise onto the face of each diagonal and crossmember when they were dry fitted in place.
Balancing a heavy 1/2" router onto a 1" wide board is not easy so I ganged a piece of stock onto the side of each piece with clamps. This gives a wider bearing surface. I also used a shop made plexiglass base screwed onto the router to fill in the cutter void and increase its length.
Clamps at either end to create end stops.
The resulting mortises obviously have rounded ends. Loose tenons were made to be a hair shorter than 50mm (2") and each edge had a 1/4" routed radius to match the mortises.
The loose tenons were glued into each of the diagonals to aid assembly further on in the process.