That's an excellent way to start. The Art of Manliness isn't a bad tutorial, but it might have been a one-off for the website. I think you might find better (or additional) information using the search option on this forum. Look up sheaths and study the ones you like. People have also posted progress pictures, so you can see it step by step. Youtube is another good option. I'm sure there are a bunch of really awesome sheath makers who post how-to videos.
If you watch Nigel Armitage's video on saddle stitch (https://www.youtube.com/watch?v=TGuiha5S2oE), he goes into detail about needles, thread sizes, etc. He has several videos on the subject.
I would recommend .8 mm tiger thread and harness needles (you can get them from tandy, or probably anywhere). I got my tiger thread from Ludlow leather, and it's about the same price as tandy thread, and lasts for a very long time. I don't make sheaths, however. Your best bet is to search the forum, and see what size thread other sheath makers use.
Diamond sewing chisels make it easier to get a consistent look. You can use either one.