This is a simple and easy project to add a leather wedge to your Korean Male thumb ring. Really this project only requires a knife, cutting surface, and a ring but a straight edge, cutting mat, and rotary knife can make getting clean results a little easier.
Step one is laying things out. You'll want leather which is about 1-1.5mm thick and reasonably soft/flexible. You'll want a strip about twice to 2.5 times as long as the ring, and of equal width. This is then to be tapered so that at its thin end it is 1/3rd the width as it's thick end.
At the thick end cut a slot into which the thin end may be inserted. Pass the leather through the ring, and then insert the thin end into said slot. The result should form a loop. The overlay of thin end onto thick can be used to further increase the "wedge" capability of the leather. This loop shape can now be secured either with stitching, or a little leather adhesive.
And just like that, you're done. Rotate the loop to the narrow part in order to insert you thumb in the ring. Once in position slide the loop around until the leather comfortably secures your thumb in place. Easy as that.