This button can be used to finish a square or rounded knot braiding. It can also be done with more cords than the four I have used, but I'll show it with four to make it easier to follow. Also, of course, it can be done with leather lacing, and I think it looks beautiful when done that way. I use it alot on my leather lanyards.
It's useful to have a marlin spike to open the knots when needed , although it can be done with a rounded awl or even a nail, but you will have a lot more trouble that way

In this pic we can see a knife with a fob made with 4 strings round braiding that need a button to finish


The first phase of the button is completed doing a square knot, passing one cord over the next on the right



The second phase is achieved by doing a second square knot passing one cord below the next one on the right.



To finish the button you must open the knots in such a way that every cord can pass below the next one on the right and be and go out throught the center of the first knot


Then you just have to tighten the knot and trim the tips of the cords (or not) and it's done






I hope you'll have fun doing this one
Esteban