OK, a deeper pouch-type sheath - the pattern was made pretty much as described above by camano ridge:
Big hint with wet-moulding around a knife - grease the blade with Vaseline or silicone grease, wrap in cling film, then tape over the cling film.
Saves loads of rust spots!
For your Green River style knife, I would suggest a modification of cr's method above:
1. Draw round the knife on a sheet of graph paper, extending your pattern up as far as you want the knife to sit into the sheath.
2. Expand your pattern laterally so that the width across each segment of the blade is twice the blade width, plus suitable seam allowance (I normally work on 1/4" per side).
3. The sheath will need to be wider around the handle - measure the circumference of the handle at a few heights, and add your seam allowance again - this will be the width of your pattern around the handle.
4. Fold in half and stitch the seam together.
5. Soak it!
6. Re-fold the sheath flat so that the seam is in the middle of the back surface, then push your wrapped knife into the sheath, making sure the seam stays straight.
7. Allow to partially dry, then hammer the back seam onto the knife with a light, CLEAN hammer.
8. Allow to dry completely, but remove the knife before completely dry.
Finish the surface according to your preferred method.
This type of sheath does require a frog, or if you make the back seam a little deeper at the top, you can use a D ring to make a dangler-type sheath.
HTH