I agree with Richard, contact cement!!!
I always made my own leather discs and punched a hole in them and put em on a piece of all thread with nuts and fender washers to clamp them together good for a couple days till I was ready to assemble the knife. This also allows you a handle so you can preshape the leather before putting the knife together. Once I got the handle shaped the way I wanted it, I sanded it down to 320 grit and them buffed it with white rouge on a loose sewn muslin wheel. This will not only make it shine a little, it will slightly burn the leather, leaving it a nice brown color. Hope this helps some!!!