I've posted a few sheaths here, and none of them are of extreme pro quality, but they all work for me.
I'm just learning with kydex, so my best advice there is to build a press, watch a few itube videos, and experiment with a couple of your user knives. I'm using a toaster oven, but would like to supplement that with a heat gun. Again, my best advice here is to build a press.
I have slightly more experience with leather, but not a whole lot more. Get a piece of leather substantially larger than you think you'll need, and cut it so that one side is flat.Save the trimmings Put the knife down on the inside of the leather, and move the curved side of the knife over towards the flat edge, stopping about 1 inch from the edge. use a compass or a scribe to mark a line roughly following the curve of the edge leaving about 1" of space, and trim along that line. Mark a straight line down the spine side of the knife, and make a LIGHT cut along that line. This cut is only to make the leather easier to bend, and you can leave it out of thin or soft leather.
Now fold the leather along the spine line. Mark where the edge cut you made meets the leather, and trim. Use your compass to draw a line about 1/2" from the edge, and use a punch to make holes. There are numerous tools to do this... tandy makes one that punches 8 slots at once, or you can use a tracing wheel from a sewing store.
Take a piece of the last trimming you made and use contact cement to glue it to the inside of the rough sheath you now have. This will become your welt. Do the same on the top of the welt, and clamp everything closed for an hour or two.
After the glue has set, punch holes. If the leather is thick and stiff, a drill press works great here.
Mark and cut out belt loops and restraining devices, and sew them in now.
Sew everything up using proper thread.
5-6 ounce leather is on the light side, ok for small knives.
7-9 is best if you want to tool, or for larger knives.
Cour Boulli (more often seen in search engines as courboulli) is leather boiled in wax. This will both stiffen and shrink the leather. You can also form-fit the sheath at this point.
You can also form-fit by soaking the leather in water instead of hot wax.
You've made your first sheath, and along the way you've made a bunch of errors, done a lot of things you wish you'd done differently, changes you'd make on your next sheath.
So, ok. the sheath you just made isn't one you want to take to the gun shows, but it will work in your truck or tackle box fine. And you learned a ton about what to do next time.
Now apply that knowledge and make your second sheath. This one will be better, and you'll learn even more.
Again. I am a raw beginner, but the sheaths I am making now are functional and I'm not ashamed of them. A couple of them are even pretty decent. I guess my best advice is just make a couple; be willing to screw up, but learn from your mistakes.