CARDBOARD STAYS MOCKUP
Have you been introduced to the methods and virtues of making a wearable mockup in corrugated cardboard as a first step in making new stays?
Making stays is time consuming, and a penance if they end up not fitting well. A "muslin" or mockup needs to be boned before it can give you anything like a fair idea of how the final stays will fit; you practically have to make your mockup into fully finished stays to learn anything useful from them. That's a lot of work. Enter the cardboard mockup!
I had a reenacting friend, a lady of generous proportions, who made her cardboard mockup, and wore them to an event as a trial ... and didn't bother with actual stays. At least, not for a good long time. She was so happy with her cardboard stays that for an entire reenacting season she just wore them. For the few times that year she went to an event, they served her adequately well. By the time the following season rolled around, she had her real stays done, but for that first season, the cardboard stays worked fine for her, and no one ever caught on.
Cardboard will never give you a perfect fit, it's just a good quick-n-dirty, wearable, approximation, but as a fitting tool, it's a trick worth knowing, and one that can save you a lot of trouble when you put a ton of time and effort into making a 'real' pair of stays.
You will need: Stays pattern, clean corrugated cardboard carton sliced open and laid flat, pen, pencil or sharpie, box cutter or utility scissors, duct tape, paper hole punch, and sturdy string.
Select the stays pattern of your choice and make any obvious alterations you think you'll need, such as torso length. Transfer the pattern to the cardboard, eliminating all seam allowances, and aligning the corrugations in the cardboard parallel to the direction of the boning channels in that section of the stays. Cut out the sections or panels of your intended stays from the cardboard. Then duct-tape any place where the cardboard might tear, such as along the edge where it laces. Put duct-tape at the tops of the slashes at the waist tabs ("fingers") so they don't tear upward. Tape the underarm curve. Then tape all the pieces together at the seams. And then burnish all the tape with a thumb nail or the handle of your scissors so it is well adhered.
Once you've assembled the cardboard mockup, you will want to roll it gently before trying to bend it around your body. Otherwise, instead of curving, it may decide to crack and fold at just one point. Punch the lacing holes with your paper punch, and then use disposable string to lace it, because the duct tape along that edge may well gum up the string. If your pattern calls for straps, use string in place of them.
I encourage you to make it such that your cardboard has at least a 2" gap in the back, no less, because the fabric ones will fit closer to your body, and they will also have some stretch to them that the cardboard doesn't have.
Try it on, wear it for a few hours. Do the tops of the slashes sit exactly at your natural waist, no higher and no lower? Does the underarm curve just touch your tendon at the front of your armpit, or does it dig in painfully? Is the center front edge the right height, about an inch above the nipples? Can you perform all necessary bathroom functions? A quarter inch shaved off or added here and there can make a huge difference. It's easy to do now, but much more difficult once your stays are sewn, boned, and bound.
Then put it on again the next day. That's when you really discover the sore spots you weren't even aware of, the ones that need fixing just a little, for the difference between stays you can stand to wear well-enough, as opposed to stays that are as comfortable as a hug.
When your cardboard stays fit you that well, you are ready to make them up in fabric. Use your corrected cardboard pieces for your pattern, and don't forget to add back in whatever seam allowances it requires. If you choose the correct fabrics and boning materials, you are almost guaranteed a well-fitting pair of stays as a result.