When you want to sew the seam on the side of a leather scabbard for a knife, bayonet or sword; there were two main authentic ways dating back to the late 17th century throughout the 18th and 19th century. For both of them a wood form was usually made to replicate the blade as already mentioned. Nowadays we can put a thick layer of gun grease on the blade, wrap it in Saran Wrap. Then add either some slivers of wood or leather around the blade and wrap masking tape around that to ensure the scabbard will fit correctly. Then you wrap masking tape aound that to hold everything in place. I usally do it the latter way as I don't make enough of these types of scabbards to make it worthwhile to make wood forms.
One way to do it is called a "standing butt seam." You wet the leather and wrap it around the form. Pinch the ends in some kind of holder or vice very close to the form and down the center of the form on what will be the back side of the sheath. The ends of the leather come together and go upwards from where they were pinched together. While the leather is wet, you sew the seams together with a standard double stitch as close to the body of the form as possible. You re-wet the leather as needed as you go along with a sponge. Then you cut the leather above the seams very close to the seams and burnish them down while the leather is still moist.
You wind up with a seam that looks something like the one on this bayonet scabbard, though they didn't burnish the leather down
http://www.thequartermastergeneral.com/?category=leather_goods&morepics=true&id=811I don't mean to be a snob, but this type of seam often was not done well and was cheaper and easier to make. It was not considered "the best work" in the 18th century. This type of seam is very uncommon on sword scabbards from the mid 18th century to the Indian Wars period.
What was considered the "best work" was to wrap the leather around the form with both ends of the leather overlapping the center of the blade. Then cut through both pieces of the leather. at the same time, down the center of the back side of the sheath. That gives you a near perfect mating of the leather ends so they are flat on the blade when you use tunnel stitches to sew the flat ends of the leather together. You have to use a curved awl to make tunnel stitches so the ends of the leather will lay flat and end to end. Here are some examples of what that kind of seam looks like:
Cutlas scabbard: http://www.horsesoldier.com/catalog/185-12c.JPEG
Bayonet scabbard: http://www.horsesoldier.com/catalog/30-1491b.JPEG
Though these examples are from the 19th century, it was done the same way in the 18th century.
We had quite a good discussion on the British Blades forum about how I found out how to make this stitch from the Cordwainer at Colonial Williamsburg and how to actually do it. You have to go to the second page in the link for the information. The first page deals a lot with how original French and Indian War bayonet frogs were made. (I'm known as U.S. Marine on that forum)
http://www.britishblades.com/forums/showthread.php?t=79432For anyone who wishes to do handsewing of leather work, there is absolutely one book that is worth many times what you will pay for it. The book is:
The Art of Handsewing Leather, by Al Stohlman. It is available from Tandy Leather and on Ebay as well as other places. One caution though for doing this kind of sewing on scabbards when you look at the pictures in the thread from Al's book. You don't want to make a stitching groove along the ends of the leather as that isn't authentic.
If you have other questions, please ask.