1. Cut a strip of bark aproximately double the width of your blade and 4 times as long. Best to cut the strip from a thicker piece with the eyes of the bark parallel to the blade or they may crack open. For my knife, this meant cutting the strip from a large piece of bark left over from the canoe build. It had a natural split in it so I just continued the cut and cleaned up the edges with the sharp crooked knife.
2. Soak the bark in water to soften it and fold in half, then bring the the ends to the centre fold line.
3. Cut another narrower strip about the same length for the wrapping. To prevent cracking, I thinned this piece out by delaminating a couple of layers. This also lightened the colour to add a nice contrast.
4. Now the tricky part. The whole sheath is held with the tension of the wrap agains all these folded layers. Beginning at the bottom of the sheath, insert the wrapping strip on an angle and wrap around the main piece pulling tightly. After a single wrap, insert the end into the opposing folded slot and continue. You should end up with an alternating pattern that places the wrapping strip on the outside of the sheath then on the inner fold. Hopefully the pic shows this clearly
5. Trip the excess bark around the top of the sheath and tuck into the centre fold. Once the bark dries (quite quickly) it'll hold its shape quite nicely.
6. The sheathed knife. If you've selected healthy bark, it should not crack even when dry and have a little bit of springy give when removing the knife, sort of like thick leather.