I will be adding a second layer of leather over the bottom edge of the neckguard, in order to reinforce it and make it lay smoother. If I make the trim the same size as the neckguard, and stitch them together, the resulting piece will want to lay flat, which I don't want. The neckguard should roughly form a half circle. In order to get it to readily make that shape I will make the trim piece longer than the base piece.
If you take two pieces of paper—one slightly shorter than the other—and fasten the ends together, it will bend into a curve. If you wrap tape around a tube, and keep adding it on over itself, the tape will build up in thickness, with the circumference of each turn being slightly larger than the previous one. Drafting a pattern for the neckguard uses the same principle.
I can make the patterns in paper, and bend and measure them to find the right size differential, but that won't be accurate for cutting it in leather because the leather is thicker than paper, and thus needs a larger differential. Add to that the fact that vegetable tanned leather—which I cut and stitch while damp—will shrink while drying, and I can only guess at how long the trim should be.
Here is the neckguard trim piece after cutting and dying.
And here it is tacked into position. The oblong openings will hold luminiferous aether collection devices.