Reddit mentions of Simplicity Sewing Pattern 3635 Misses 18th Century Undergarments, Corset, Chemise, Panniers (Hoop Skirt)

Here are the pair I made based off of this one at the MET. These are the fourth pair of 18th C stays I've made. :-) I keep getting slightly better each time.

Do NOT listen to the others that say zip ties - what they most likely mean is Duct ties. Duct ties are awesome - and what I use- but they are thicker and come in longer lengths than zip ties. Duct ties can come in lengths up to a yard (I heard of a magical 48" long duct ties pack once but I've not personally seen it) and are available in the same section as zip ties, normally, at your favorite hardware store. You will need heavy duty scissors to cut the ties to length and you will need a nail file or one of those sandpaper cubes to file the ends so they don't poke through the fabric.

As for a pattern - I've tried the Simplicity Pattern 3635 and it came out too big - despite a fairly decent mock up. Make it at least a size smaller than you normally would because you want to be squished. As u/stringthing87 stated, JP Ryan is also a very good pattern - I've used the gown pattern from JP Ryan and love it.