They're all different depending on various measurements of blade, handle, girth, and on your perception of what makes a nice sheath.
Cut the sides out of a cereal packet and wrap your knife in them and start trimming off anything that isn't sheath-shaped. When you get a shape that you like, transfer it to leather and off you go. You'll have to include a welt and a firesteel loop if they are your thing.
ATB
Ogri the trog