Step 1: Find a cheap dress + make it fit
Doing all the fitting took more skill than I have, so I took it to my friend Miss Bombshell in Oakland. She cut away the excess fabric, sewed up the centre, gave it a V-back, and put in some boning. I found some matching silk in a fabric store bargain bin, and she sewed that into a sash.
(Remaking dresses is challenging, but here are a couple of Instructables that involved some impressive surgery: Custom Wedding Dress and Upcycled Dream Wedding Dress & Accessories.)