Take one end of your string and tie a loose knot
Slide all your bobby pins through the other end
Pull one end through the loose knot and make another knot around the other side.
Tighten both knots and pull at the two sides until the knots come together . Separate the knots and adjust to ur desired length
|Leather Cord Version|
Note: To get a different look you can also spray paint the pins. I'll post some pictures of spray painted ones on a later date.