The boning across the belly will be contained in pre-made bone casing tape. The tape is available by the yard in different widths and in black or white. I'm using 1/2 inch wide black tape.
I basted each casing in place, folding the open end under at one end, but leaving the other open so I can slide the boning in. Then I laid artist's tape across the front as a guide for stitching. Even though I used low tack tape I did not want the adhesive pulling the brocade so I stuck it to my jeans a few times before putting on the corselet.
Since the basting was at the top of the casing I sewed the across the bottom of the casing first. I don't know if It would have made a difference either had I sewed across the top first. Probably not. After the first line of stitching was done I sewed the second right on the basting stitches.
The first casing from the front.
And from the back.