What you need:
Buttons with 4 holes
Colored Hemp Cord
Cut 4, 12 in pieces of cording. You can do 2 one color and 2 a different color, or just all the same.
Thread them through one button hole at a time tying a normal knot with both ends. Do all four and then tie the ends or add a clasp. If you tie them you may want to glue them with something like E-6000. Have fun!