I’ve been known to be a little impatient. So, instead of waiting for the finishing instructions for the 15-sided biscornu to be posted, I forged ahead on my own:

15-Sided Biscornu from Kathrin Ellison
The beads are Mill Hill Magnifica colour 10020 (Royal Amethyst). The DMC colours are 208 and 209, the over-dyed is Needle Necessities 1481 Mystique; and fabric is 28 count linen from Enchanting Lair’s in colour Ametrine.
I understand that Kathrin is planning on using beads as part of the finishing. Unfortunately, I don’t have very many beads left; perhaps only enough to do half of the part where you attach the last five squares. I don’t think Mill Hill gives you as many Magnifica beads as they do regular seed beads. Which is a pity, since they cost so much.
I haven’t attached buttons or beads to the centre yet, since I haven’t found the right ones. But, otherwise, I’m very happy with how it turned out. I’m not in a rush to do another 15-sided biscornu though, since the finishing takes much too long.