I've never seen one of those in the flesh, but I'll take a wild guess:

. . . Put the blade in its sleeve,
. . . strop it on a piece of flat leather (or tight strop) as though it were a little piece of
. . . a straight razor.

If the thickness of the sleeve (at the base of the blade) is about 1/3 of the width of the blade, that will give you a decent angle at the edge.

Charles