The foundation

In fact, I'm pretty pleased with my progress so far, even though I haven't constructed any part of the actual dress yet. I'm getting a pretty clear idea about how I'm going to make it and that makes a huge difference.

I've started at the beginning: By making the fitted bodice with bra-style cups which forms the basis for this design. 

This is a bit of a cross-over between clothing and lingerie patterns so I thought I might talk you through the process I used.
The cup pattern pieces come from my normal bra patterns. A horizontally seamed cup for a strapless design with a curved top edge.
I drafted the bodice from scratch. That seemed easier than fiddling with a body wear sloper with negative and a normal body sloper with positive ease.

To do this you need just a few measurements: 
- Waist circumference
- Distance from the waist to the underwire of your bra. To measure this, tie a ribbon or a piece of elastic around your waist, tightly so it automatically comes to rest at the narrowest point, and measure up from there.
- Underbust circumference
- Then choose how far the bodice to reach below your waist and measure your body's circumference at that level (the ribbon is helpful with this as well)

Then, you can draft the pattern like this:

- Draw a horizontal line the length of half the 'hip' measurement and square up from either end and the middle. Also draw in the waist and underbust height.
- Place the center front of your bra band pattern along the center front line and the bottom of the curve for the cup on the underbust line and trace the bridge, cup curve and beginning of the top of the band.
- Mark 1/4 of the underbust circumference from either side on the underbust line. Square up to the tracing of the bra band and shape the top edge of the bodice.
- For the waist shaping, divide the waist measurement by 2 (because we are working on a half pattern here) and then by 12. Take out 3/12 on either side of the center line, this will become the side seam. Then take out 2/12 in the front panel and 4/12 in the back.
- Connect all the points you've measured out and don't forget to mark the straight grain (perpendicular to the waistline). When cutting, mark the waistline on all pieces and match those marks when sewing.

Of course, a pattern like this may need quite a bit of fine-tuning so I decided to sew it up to try the fit, using the fabric I plan to use for this layer. 
I made the cups from lingerie foam (I think you have to use a material which can stand up on its own for this shape) in my usual way and sewed them into the bodice. 
For fitting purposes, I put a separating zipper at the back.

And this is the fit, with underwires but without boning (and pushed up weirdly at the bottom because of the fabric I put around my hips for the picture). Not too bad, if I say so myself. Later, I sewed poly boning to half of it and it looked even better (I plan on using spiral steel for the real thing). 
Based on this try-out I've made a few changes: I cut the pieces with seam allowance on the bottom but I like the length like this, so I will add 1.5 cm. I really didn't like the way the cup and the top of the bodice look like completely different things where they meet at the side. I raised that point by 1 cm to make them join more smoothly. And finally, I shaved of a couple of millimeters at and just below the waist at the side seam and front dart, because it was a bit loose in those places. 
After that, I spend a lot of time thinking about the order of construction and the way to finish this thing, with the draped fabric on top, neatly. But more about that later.   

P.S. This would also work well as a pattern for a lingerie corset. Just make the center back panel a bit wider compared to the side back, then narrow it about 10% for negative ease. Cut that part out of power net and use something like sturdy satin or lace over non-stretch net for the rest.