Hi there. First time post for me. Been reading and learning things here for a bit, off and on.
Nice looking sheath you have created.
In regards to the welt, I found this page helpful, http://beebeknives.com/html/make_a_sheath_tutorial.html , and this page/image, http://beebeknives.com/html/make_a_sheath_tutorial_-_details_19.html , as details for the welt.
The welt is stitched or riveted (if you want to use them, I've noticed most here do not), between the front and the back of the sheath, and is just a 1/4" strip that runs the length of the edge of the blade side of the sheath. This gives the blade an edge to touch, without fear of cutting the stitching, or the blade hitting the rivets and dulling or chipping the blade.
Good luck, good crafting, and enjoy!
Ronnie