For durability, you want a feather that's not too soft, so that you won't be resharpening it every ten minutes. You also want one that's not too flexible; if it is too bendy it will feel more like a paintbrush than a pen, which is sort of annoying - and not too thin, so that you can grip it comfortably. Primary flight feathers are good for quills. Secondaries aren't so bad either, so long as they're big enough. Other feathers tend to be too narrow.
When I'm cutting a quill the first thing I do is chop off most of its length and all of the fluffy bits, so that what remains looks a lot like...a pen. Surprise. Then I soak it in water for ten minutes or so, which makes the next stages easier: chop off the very tip, use a small crochet hook to pull all the crud out from the inside. scrape the membrane off the outside, and roughly shape the end so's it looks like a nib.
Then sometimes I temper it, but not always because I haven't got this down reliably yet, and sometimes it doesn't seem to make a lot of difference. The idea with tempering is to dry the natural moisture out of the feather, so that it will stay sharp longer, by judicious application of heat, chemicals, and/or time.
Then I cut the channel up the middle. The channel is super-clever. It works like the bristles on a paintbrush, to hold a small supply of ink and let it out as needed. If you don't have a channel you can write maybe one or two letters before you have to get more ink. With a channel, you can write at least four or five, and maybe twenty or thirty. People brought up on ballpens and fibre-tips usually think the channel means the nib is broken.
I use a razor blade to cut my channels, but other people swear by knives or by squeezing the feather till it cracks. You do what works for you.
Then I use a scalpel to do the fine-shaping of the nib. A proper proper scribe would use a pen-knife, which they would sharpen periodically. I'm rubbish at sharpening knives, and a knife made of really good steel is expensive, so I use surgical scalpels, which are very sharp for a short time, but aren't designed to stay sharp so I have to keep replacing the blade. I feel a bit bad about this.