1 - design a yoke that fits over your corset
2 - figure out how many layers to attach to the yoke
I started with the yoke, and assumed that I would want the waist about an inch bigger than my corseted measurements but with a couple of different bars on my hook and bar closure. That accounts for size variation. Which, you know, sometimes happens. Then I added 2 inches to my corseted hip measurement. That's what my finished size should be. I planned on two front and two back pieces, with the bulk (but not all) of the 'spread' occurring in the sides and back. I dug out some canvas scraps to make the yoke. I had enough to cut the two backs but had to piece the two front sections.
From now on, this petticoat is named Sally, after the heroine of Nightmare Before Christmas. It's a Frankenstein of pieces. It's also going to be worn UNDER a dress. We all save scraps for a future use. I've finally found one. In addition, I found some muslin to use on the other side (that will be against my corset) and wrapped it around to finish the edges. Then I used twill tape as a waistband. Here's the nearly finished yoke (yes, yes, it's not so easy to see and I'm lazy).
Now that I have a yoke, I need to start cording. My original plan was to have 4 panels that tapered from the bottom up to the top but after a lot of internet searching, I've decided to use a big rectangle. Cording on an angle sounded like a disaster waiting to happen.
I had hoped to be able to each each row as a guideline for the next, but (as you can see in the picture) my rows start to go wrong. Not a lot, but wrong. I realized that I needed to stop guesstimating and mark my fabric before the rows got so crooked that I had to re-do them.
It took a long time and the lines managed to get crooked too!
Still, they're better than nothing at all. Then follows the long and tedious task of sewing in the cords. All 35 of them.
I can see the light at the end of the tulle tunnel! The muslin ruffles are cut and need to be hemmed. Then I'll sew a cord thru the top and use that to gather them to the petticoat. I plan to have the muslin overlap to help with the smoothing.
Once I'm done with this layer I'm going to sew it to the yoke, over the corded layer. I'm waiting to hem the cored layer until everything else is done. Then, if I want even more body, I can add stiff interfacing or horsehair canvas to the hem of the corded layer.