Mostly I relied on these videos:
https://www.youtube.com/watch?v=qdE8ihW5qbs
https://www.youtube.com/watch?v=Qt8BnF7zE-Y
(I dislike videos in general, but they’re helpful for something like this.)
It seems like the basic procedure is to start by making a long shallow cut lengthwise, to get something you can write with. (Or if you’re a weakling like me, whittle bits away until you get to something which looks vaguely OK.) Then turn it around and slice away the back, to make the tip flat on both sides. Then you can square off the top of the nib and trim the sides until you get to the width you want. It took me a few tries to get it right, but now I’d say I can make one in only a couple of minutes.