Heres my 'How To...'
Sorry theres no pics, my version of XP has decided that my camera is un-acceptable :borgsmile
I'm going to assume that your blade is more or less straight (ie not a Kukri)
First of all find yourself some boards, hard-woods are best, riven rather then sawn if you can get it
What you're looking for are boards over an inch (to two inches) wider then the widest part of the blade and one and a half to two inches longer.
Thickness is the next thing, you can either have two or three boards.
If you're using two boards you'll need one 1/8 inch thick for every nine inches (or part thereof) of the blade(calling this A) and the second this thickness plus the maximum thickness of the blade plus a bit, I use 1/16 of an inch (calling this B)
If you're going with three boards you'll need A, another A and C (which is a board equal to the maximum thickness of the blade plus 1/16)
Marking up
Decide which side of each board is going to end up as show-wood (A and B only )
Decide which end the 'mouth' is going to be (if your bolster is not at right angles to the blade this is when you need to cut the boards to match)
Mark a line half an inch in from one edge of A (show side only) and B (both sides, but the same edge), or both A's show side and C any side
Lay the blade on the wrong side of B (or one side of C) with the back edge laying along the line, push the bolster hard against the mouth edge (the blade should remain on the line, if not pare the mouth edge away untill it does)
Take a pencil (the carpenter's type is best, but any will do) and holding the blade as flat as you can and the pencil as upright as possible draw around the cutting edge (there should be an 1/8 of an inch gap between the edge and the line, this is important)
Hold the blade flat on the board and slide the back edge along the line (as if you're drawing the blade), mark any place the cutting edge crosses the line you just drew (this is important with blades that have a wider point ), keep checking the draw untill the blade will clear the sheath without crossing the outline
Repeat this on A (or both A's.
Cutting the housing
Time to use your carving skills, you need to cut a flat slot the depth of which equals slightly more then your blades maximum thickness into board B, you've just marked the outline. I use a small carving burr in a dremel clone to trace the outline and then a carving gouge and a chisel (I have done this using an angle grinder....but I was young and stupid at the time). Make sure the corners are sharp
If you're using three boards, cut the outlined piece out of C with a fret saw or jig saw, or whatever you do have
Fixing
Line up the edges where you put the first line and glue the boards together making sure that the mouth matches on your boards
Personally I use Araldite (the standard cure not Rapid), I've found that you must be fairly mean with the glue or else you get a bit inside your box and you then can't get your blade in.....Do not let the glue cure with the blade in the sheath, it can get stuck
Shaping up
You now have a hollow bit of wood with the outline of the slot marked on both outsides.
Draw the outer edge of your sheath now, leave at least 1/2 an inch from the slot profile on the cutting edge and 3/4 at the point
If you want to have a retaining thong, mark a bump out on the cutting edge side and drill a hole now
Cut your outer edge and shape (please note on a two board pattern the glueline is not down the middle)
Finish in a pleasing way
Hows that?