The following is a tutorial on hand sewing leather using a method I have
developed over forty years of seeking the "Perfect Stitch". It works for me.

My first suggestion is to get the book "The Art of Handsewing Leather" by Al Stohlman.
It's available from Tandy Leather Company or the Leather Factory amongst others. Read it and learn.
Some of the differences from the book and the following method are due to arthritis in my hands,
so you can see I'm looking for the easiest yet best way of doing this.

This picture shows the set of tools I use for sewing.

Top Left: 5 cord linen thread Tools on left (clockwise from left): Beeswax/Pitch mix Stitch Groover Stitch marker /Oversttich Wheel Diamond tip awl used with Hammer Western style edge rounder (Weaver Leather master tool) In the center is a home made stitch pony Top right:       1. Round tip awl for opening up holes for backstitching       2. Diamond tip hand awl One of a pair of gloves - the little finger is left on to prevent cutting yourself when sewing

1) Do whatever decoration you're going to do on the face of the sheath whether a foldover or two seam style. I make the back side and welt of my sheaths slightly wider than the face. That way when I cement the whole thing together before sewing, the back and welt stick out slightly around the face.

2) Groove for the face seam only. (all expert hand sewers do this and it doesn't weaken the leather as long as you don't cut too deep). It will help keep your stitches even and also helps set the stitches below the face so the thread won't wear as easily.

3) Lightly dampen the stitch groove and then use a stitch mark wheel and mark your stitches (if you get the leather too wet let it dry until almost the same color as the fully dry leather). For sheath construction (6 - 10oz leather is the norm - I use 8/9 oz most of the time - an ounce of leather equals 1/64" of an inch ). For stitching I use a 6 stitch per inch wheel because I think it works and looks best with 5 cord thread. For decorative stitching, such as around inlays, I use 7 or 8 spi and use 3 cord thread.

4) Make a single hole punch from a diamond awl blade and polish it on a firm buffing wheel using red rouge (As an alternative to making your own, you can purchase an Osborne #143 peg awl haft which will give double duty as both a hammered punch and a hand awl. It will also allow the replacement of your awl blade if it gets broken). Using a light non-metallic hammer and this punch, punch all the holes IN THE FACE ONLY. Make sure when punching the holes that they are are lined up properly as illustrated below. Keep a block of beeswax handy and wax the awl frequently; this helps punch cleaner holes and makes it easier to pull it up and out. A slight twist should be all it takes to remove your awl blade.

The above image is from "The Art of Handsewing Leather" by Al Stohlman.

I use a double thick "Silent Poundo" rubber board as a backing when punching the holes. I use a double thickness to make sure I don't bend the point over against the hard table top. Also you don't have to hit real hard, just enough to ease the blade through the leather.

5 ) After punching the holes in the face I glue everything together and let it dry overnight. It's best not to sand or sew until you've let the glue dry completely (voice of experience!). I then use a belt sander with a clean 80 grit belt and sand the edges flat and perpindicular, barely touching the front face. Take it slow and easy so you don't burn the leather. You can also trim the back/welt very carefully with a SHARP knife and then use a chisel-edge bladed knife, and while holding it flat use it like a draw knife to get the edge smooth and even.

6) After smoothing/evening the edges reset your stitch groover to match exactly the groove on the face. Next run your groover around the back side. Then put the sheath in a stitching pony or horse (the book above shows how to make a horse that can be clamped to a chair or you can buy one from several different sources). If you glue some thin leather to the face of the jaws of the horse/pony it helps keep from marking your leather. Here is a link to the WRTC Stitching Pony Tutorial.
DON'T OVER CLAMP! - just want to hold the item steady between the jaws with the seam and about 1/8"-1/4" extra above the jaws. Experience will tell you what's right. Now take your hand awl and using the holes punched in the face as a guide, push your awl through the welt and back. Keep the awl blade well polished and wax it frequently. Using this method I can punch an awl through about six layers of 8 oz vegtan with relative ease. Push slowly while watching the back side and if the awl tip starts coming out crooked pull back and adjust it so that it will come out properly spaced and in the groove. After all the holes are punched front to back you then turn the sheath around and push through in the opposite direction - this "eases" the holes on the back side.

7) Take the leather out of the horse and LIGHTLY dampen the seam(s). Then carefully run your stitch wheel along the seams to line up the holes on both the front and back of your sheath - take your time and Be Careful. This extra operation really helps even the stitch line. I let the seams dry before sewing. Some people swear by sewing when damp, but I have found with the weight of leather and stitch length I use that you will tear out stitches more readily.

8) For sewing I use the time honored two needle "saddle stitch" method with waxed linen thread (I buy unwaxed thread and wax it myself). I use 5 cord linen which has a 35lb tensile strength, but if you want heavier you can get 7 cord (I don't see the necessity and I've sewn a LOT of sheaths and holsters and have never had one break out at the seams unless abused - but each to their own). I use Barbour's Red Hand Linen Thread which you can get from Mid-Continent Leather Supply 800-926-2061. The thread comes in about a 1000 yd spool (5 strand) for around $35.00 and it will probably last the average leather worker a life time. For the 5 cord thread I use size "1" or "0" harness needles. If I'm using 7 cord I get the "00" size. If you're just starting out you can get a pretty good starter kit from Tandy Leather or Leather Factory which comes with all the basic stitching items necessary including the book for around $29.00.
Good linen thread only comes in natural so if you want it darker cut off a length (I use a length about 1 1/2 - 2 times longer than the seam I'm sewing, but it never hurts to go longer) and dip it in a shade darker leather dye than the sheath. Pull it through a soft cloth to clean off the excess dye and let it dry.
To wax my thread I use a trick taught to me by a shoemaker. I make up my own version of "hard" wax. I take a pound of beeswax and a pound of pine pitch (you can get both at Jas Townsend and Sons - www.jastown.com on the net). Melt them together in an old crockpot or double boiler (this is VERY flammable so be careful). Let it cool slightly and then pour the mixture into some ice cube trays (aluminum is best if you can find them) for convenient sized blocks. After the blocks solidify, cool the tray(s) in the fridge and the blocks will pop right out. This mixture makes the thread "stickier" so when you pull it up snug it locks the stitch tightly into place and it won't "bag". To wax my thread, I fold the length in half and hook it over a smooth hook attached to the wall. I then rub up and down both strands equally (make sure you get the center section too) with the wax until it's well coated, but not over coated. Then I take a piece of buckskin or brown paper bag and rub vigorously up and down the thread. The heat from the friction "melts" the wax, forcing it deeper into the individual fibers.

The above image is from "The Art of Handsewing Leather" by Al Stohlman.

9) I start my seam with three back stitches and end it with 2 1/2 (back stitch the top thread three stitches and then cut both threads off on the backside). When pulling your stitches tight don't yard on your stitches - just pull snug - firmly and evenly. Otherwise two things may happen: You will either break your thread or you will tear a stitch. Actually, if you sew long enough both of these things will probably happen and in a future tutorial I will explain how to "fix" them without having to start your project all over again. After you're done sewing, tap over the seams lightly with a slightly convex face mallet or hammer (they make special hammers for this, but a good steel carpenters or ball peen hammer with the edge rounded will work fine). Then LIGHTLY dampen the seams and run your overstitch wheel carefully up and down the seams front and back. This is the final process for sewing that really evens and burnishes the stitches giving you that "professional" finish.

When done this is how your stitching should look.

11) Using your belt sander, sand the edges LIGHTLY with a worn out 150-220 grit belt or use the draw knife technique noted above to smooth the edge(s) again. Then take your edge rounding tool (#1 or #2 works best) and trim all of the edges. I edge slick by dampening the edge slightly and then use a piece of coarse cloth (worn denim works great) first and and rub vigorously up and down the edge until smooth. Then I dampen lightly again and then take a smooth elk antler tip (bone folders or a commercial slicker work well too) and rub the edge. Let it dry and then apply a THIN coat of Gum Tragacanth and while still damp rub it in with a coarse cloth. Again let it dry and follow with a second thin coat of Gum T and slick it with the elk antler.

12) To finish everything off I apply whatever water resistant finish (I like Fiebings Leather Balm or Tan Cote) you want and Voila your done!

Any way that's it for now - good sewing!