I've finished my 1920s step-in!
No, you don't get to see me in it. It's SHEER, man.
It's made of white cotton voile, with more of the pink silk ribbon for straps. I cut out a few roses out of a cheap piece lace to add over the side slits.
It's essentially constructed as two rectangles, with two pleats at the front to take it in at the top. I then made a little crotch piece that is attached at the back and buttons at the front. So from the front it looks like a slip, and from the back a sort of romper-onesie. I wear it over my 20s bandeau, and it basically removes any figure I have before. It's also super fun and comfy! I'm very tempted to start sleeping in it.