At this point my construction posts will all be super boring, because I’m pretty much following Katherine’s tutorial in it’s entirety. With credit as well to some other dress diaries whose pictures I have been examining minutely (A Fractured Fairytale, Demode). Also Janet Arnold. She’s pretty cool.
The robe a la francaise starts with a fitting lining which the dress is draped upon. I forced by stays onto my form, and started draping the lining. I figured that would be easier than scaling something up, since it’s not a super complicated pattern.

Pin fabric in a straight line down the center of the back. Smooth around to the side, pinning as you go.

Pin the front in a line from the shoulder down, vaguely where you would expect the front of a dress to end.

After taking it off the form and trying it on, I decided the fronts were way too far apart, and pinned in a piece of fabric to make it wider.
At this point, I spent something like 30 minutes in front of the mirror trying to figure out the right line across the front. You can see just how many pencil lines I tried. After way too long had passed, I said screw it and just chose something, since this will be covered by the robing anyways and really doesn’t matter at all. Sometimes I am not so smart.
To construct this, I sewed the top back for around 4 inches. I folded the seam allowance over twice to make a channel into which I inserted boning. The side seam and shoulder are all flat-felled, to hide the raw edges. I was a little crazy and only left myself 1/4 inch seam allowance, which means my felled seam ended up only 1/8 inch. Had to do the second part of each of the seams by hand, since I can’t control a 1/8 inch seam on my machine.
For the back closure, lacing is one common option. See the Janet Arnold 1745 pet-en-l’air, Interestingly this lacing is often set like victorian corset lacing (or shoelaces), and not spiral lacing.
But, ties are also period.

How amazing is the mishmash of lining fabrics? I wish I had amazing striped linen to use as linings, but there is only plain white linen in the stash.
And after sewing 50+ eyelets on my stays, I was so friggin sick of eyelets. So ties it was.
And lining is done! Now for the terrifying part – draping the sacque dun dun dun