A center seam pattern is first made as a side seam, and sewn as a side seam, although lacking a welt. Then is well wetted and twisted to a center seam. That is the easy way, but there are other methods, some using a butt joint with a special stitch and a curved needle. Articifer could better give you the details of that one. I know how it's done, but have never done it.
The way Wick (LRB) showed in his tutorial is almost without a doubt the most common way to make a period knife sheath, as that's how they made sheaths for trade knives. The sheer numbers of such sheaths meant it was most likely the most common method folks would see how a sheath was made and thus when they did it, the most copied method.
Making a sheath for a knife using the butt stitch method was not nearly as common because you almost have to have a metal tip/drag for the pointy end of the sheath and another metal throat for the top of the sheath. Some 18th century knife sheaths were made like this, but because they needed the metal throats and tips/drags, they were usually only used on high end knives, though not always.
When you want the leather to meet "flat to flat," it was/is sewn as in the following drawing:
It looks like this when the leather ends are sewn up:
You can use a straight awl to pierce the leather, BUT ONLY IF you use thick leather, as shown in the next pic.
Unfortunately, I could not find a really good pic of the more period correct way of using a CURVED awl to pierce the leather for the holes. With the curved awl, you start to piece the leather where you marked your holes, then sort of roll the awl so it comes out in the center of the leather. Here is the best pic I could find:
The top awls in this pic are the best with more curve in them for sewing leather for sheaths;
Now of course you will also need curved needles for the thread. Though you can buy curved needles separately, initially I got mine from the sewing sections in a pack like this from sewing sections in Wal Mart and fabric stores.
Oh, sometimes when the leather is not very thick, you don't need a curved awl and can pierce the holes with this type of curved needle:
I use the butt stitch mostly for sword scabbards with metal tips, bayonet scabbards and sliding loops on belts or hunting pouch straps.
Gus