This is admittedly theoretical, as there are no surviving pairs of braies that I am currently aware of. I feel I have gotten fairly close to a historically plausible pattern here, based on a review of extant documents, and images in the available Mss. of the period. They do not fit exactly as one sees in the Mss., with that skin tight spandexy kind of look, but given the materials involved, that can't really be expected. Cutting them on the bias, which I did try, does not get one significantly closed to that look, and certainly not enough to justify the additional fabric used. On the whole I have tried to stay with the period style habit of using geometric shapes where possible, and using gussets to add ease and flexibility.
These are, as noted in the original article, the most commonly seen types of braies for the early 1400 period. They consist, in my theoretical pattern, of three pieces, right leg, left leg, and crotch gusset, as seen below.
Fig 1: Pieces to be cut and measurements to be taken
Measurements: All measurements should allow for ease, and for seam allowance.
A: One quarter the measurement around the waist.
B: Desired length if the braies. I usually go a couple of inches above the knee, but shorter or longer is acceptable. I wouldn't actually go to or below the knee.. Remember to allow some for the drawstring sleeve foldover at the top.
C: Rise. This can be measured fairly closely, as the gusset will provide ease.
As to the gusset itself, I usually cut these in fairly standard sizes. For the large/Extra large person, I will cut a 6"x6" square. For medium/small, a 5"x5", and for very small persons (children and very small adults...) I will cut a 4"x4".
1. Join the legs together at the center seams. I usually leave a bit open at the front for ease of... well, use. This part I'll do a rolled hem on.
2. Join the gusset to the front of the braies. This is done by sewing one edge of the gusset from the center seam out as far as it goes along the inside leg, and doing the same on the other side.
3. Join the gusset to the rear of the braies. Done as above.
4. Sew the remaining inside leg seams.
5. Hem bottom of the legs.
6. Make sleeve for drawstring at top. I have a tendency to sew the drawstring in as I'm making the sleeve, but I don't like playing with bodkins, so do what's best for you in the matter.
In the end you should have something that, when laid out flat, looks rather like the figure below.
Fig. 2: Flat layout of finished braies.
I know it looks kinda odd, but it actually does work. I tried making the top flat, so that the legs hung straight, but that gave me a lot of bulk at the top, which, besides being on the uncomfortable side, did not lend itself to the look I was trying for. These are quite comfortable, and really pretty easy. Wear them in good health!