Cut a length of flat cord approx. 40". You may need to trim and alter the length at the end to fit your wrist better.

Thread a bicone bead onto some round leather cord.
Tie a knot and glue the knot with each end of cord sticking out horizontally. 
Thread one end of your flat leather between the bead and the knot.
Curl the flat leather under and glue.
Glue the bead to the top of the flat leather.
See image for a clearer look.

At the other end of the flat leather cord, create a loop and put a short line of glue on the outside of the shortest flat cord. 
Wrap around some round leather cord to secure the two flat leather cords together.
The long length of cord should be able to slide to make the loop bigger and smaller as an adjustable clasp, while the wrapped cord keeps the loop secure.
See images for more detail.