Choose flexible leather with a smooth finish. Here I'm using a piece that is 1.6 mm thick. Cut a circular or polygonal shaped piece of leather. If you fold it into the final position, it should be just short of wrapping all the way around your thumb.
Place a line of super glue (use the impact-resistant formula) near the center of the fold.
Press and wait for the glue to dry.
Add some extra glue if needed to make the fold hold together more solidly.
Cut a corner of leather to make a little room for the proximal thumb joint.
Just like that.
Punch a hole on each end with a 1/8" leather hole puncher.
Tie a piece of 1/8" lace through the holes.
Tie a square knot.
Here is another guard I made using the same method. The original leather is 1.6 mm thick. When folded, the guard is 3.2 mm thick. Most recommend a folded thickness of 3--4 mm.
I rarely need to retie the knot. Putting on the thumb guard is simply a matter of slipping the thumb through the opening.
I barely feel any pain when shooting a 50# bow with this thumb guard.