First, cut yourself a length of jewelry wire or cable to be used for the necklace or bracelet or other item you are going to be making. Now string a crimp bead, one side of the barrel clasp through one end of the wire, and then GO BACK THROUGH the crimp bead. But don’t go back through the clasp end again.
Some people like to string a seed bead or two behind the folded crimp as well. If you do this, then the order will be: crimp bead, seed bead, barrel clasp, and then back through the crimp bead. Same general idea. It may be a good idea to keep the folded crimp from rubbing the metal clasp end. For simplicity though, I’ve skipped the seed bead here.
Now, once you’ve strung it through the crimp bead again, push the crimp bead down toward the clasp in order to tighten up the loop. You can also lightly grasp the crimp bead with some chainnose pliers to push it down, especially if the type of wire you’ve chosen is a particularly “springy”. I’ve had different experiences with different manufacturers’ jewelry cable; some are really “limp” and don’t hold shape while others are nearly like memory wire.
You want to push the crimp bead until it is ALMOST tight but not completely tight. There should be a little bit of give, so that the clasp is still fully mobile. The reason you don’t want it really tight is that when you fold the crimp bead, it will tighten up a little more. If it is too tight then the clasp will not be movable at all.
