I should probably do a tutorial at some point.
Basically I traced the sheath onto graph paper, and then replicated this several times over. I then sketch out designs onto the paper, some of the knotwork I admit to cheating and tracing out from a book, but most of it I incoorperated into my own designs, for instance the tail on the bird at the tip was the edge of a pattern design but I transferred it onto the bird (my design) as I thought it would look nice as a tail.
The knotwork down the middle is freehand. Its easy done by lightly drawing in a diamond pattern of dots onto the sheath.
Its impossible to draw on the sheath with pencil so once Ive finished the design Im happy with I transfer it over using a sheet of graphite. You have to hold it carefully though its not easy to see if the design is straight or not and so its often easier to freehand if you can. The birds feet, faces, and internal designs on the wings were all freehand and made up as I went along.