I didn't work on a corset for myself, but helped my little sister with hers.
Simplicity Corset #2890, though with several alterations, and custom fitting.
That was the biggest learning experience of the weekend for me, the fitting, in particular the gussets.
She cuts the gusset pieces on the large size, so that we can then sew down one side of each gusset, and pin the other side in place. Then with the corset on, adjust that side for just the right fit, widening or narrowing the gusset. Of course then sew it in place. Wonderful! This had never crossed my mind, and it makes it all seem so much easier now.
Makes me want to get on to making a new one for myself! (with metal bones this time! "Grrrr" at plastic bones.)