18th century stay construction is unusual to a modern sewist because you bone each panel, sew the seam allowances down, and THEN sew the panels together, rather than sewing the panels together and then pressing down the seam allowances.

You should always fit stays over the type of undergarment you intend to wear the stays with.  A T-shirt and bra, or even a t-shirt alone, will support your bust in a very different way than a shift.  You can see how my Cassandra stays are collapsing slightly at the side, because my bra is lifting my bust inward, leaving a hollow at the sides.  

This isn’t the final sewing-together of panels: this is just a quick assembly so you can do a final fitting before the final assembly of the stays.   You’ll need to either re-sew the seams with stronger stitching if you’re using the Theatrical construction methods, or whip the channels together if you’re using the Historical constructions methods.