Here, I am using 8 1/2” x 11” paper for making my sheath pattern. As you will see later, I had to tape another piece of paper onto the top edge to complete the pattern. For this style of sheath, I draw a half pattern for the main part, as shown above, and then fold the paper along the center line to cut out the other half.
Start by drawing a straight line up the center of the paper. This represents the fold of the sheath. Then lay the knife next to the line as shown here, leaving a little space next to the fold line. Trace around the cutting edge of the knife blade and continue up along the handle, leaving a bit of extra space again particularly along the handle since it is thicker than the blade.
I don’t have a formula for working out the exact amount of space needed - allow what you think looks right, and when the pattern is all made, you can fold it around the knife itself and see what you think. Bear in mind that your leather is much stiffer and thicker than your paper pattern, so it will respond a little differently. Make the sheath to cover one half to two thirds of the length of the knife’s handle. You want your knife to go far enough into the sheath to be well secured without any straps . After drawing the inner line along the knife’s cutting edge side, measure out from that line another 9/16” to 5/8” and draw a second line, as shown. The space between these two lines represents where the welt will be.