Adjustable buckles are most often used on items like belts and bags that require changeability in their length. They are typically made of metal, with three bars through which to thread the material in question. If a buckle has been removed, lost or accidentally detached, it is quite easy to reattach or add a new one in its place.
Set the buckle upside down, with the three metal bars facing up.
Thread the strap or material through the first loop, starting underneath and threading it upward toward you.
Thread the material downward through the second loop. It will end up on the buckle's front side facing the ground.
Thread the material upward through the last loop.
Holding the buckle, pull the material's end through the loop until the desired adjustment is made.