I started into a little bit of the Regency era this summer. I made a dress first and then the stays, but decided I would show the stays first.
I ordered Simplicity 4052 from a seller on etsy and received a great pattern in great condition. It is an out of print pattern. Instead of cutting it, I traced each of the necessary pattern pieces onto tissue paper and used them for cutting my fabric. I did make a denim mock up! However, I don't think it is worth showing. The pattern fit well as is. I did narrow the straps a little and cut the front just a little wider across the front so it would be hidden under the dress.
The top layer is dupioni silk in white, left over from making my 1880s hat. The middle layer is coutil and the inner lining is white muslin. I ran out of silk so it is bound in the cotton muslin.
I used the "B" cup option for the inset gussets. It seemed to work perfectly. Of course the dress form doesn't really fit a corset but it would be indecent of me to post a picture in my undies alone!
I need to make the proper era chemise and possible a bodiced petticoat before I can wear the dress as it is sheer muslin.
Thanks for having a look.