One of the comments in my knife and multitool Toolbox column pointed to a webpage which pointed to a webpage which pointed to a posting of this nice and straightforward article from an issue of Leather Craft, on making your own knife sheaths.
[To see the rest of the article, just advance the last digit in the URL, with "-9.jpg" as the last page]
I ran across this knife sheath making tutorial awhile back. Well documented and full of useful info.
http://beknivessite2.homestead.com/pouchsheath.html