Your pup might like it best if he didn't have a license swinging from his collar, but not wearing it would make him a little outlaw in many states. To abide by the law, use an S-hook or a split O-ring to keep his license attached to his collar. A split O-ring is the same type of ring that likely holds your keys.
Open the split O-ring. As the name implies, a split O-ring is simply one ring that's split. You have two ends that open when pulled away from the ring. Opening it to slide things on, like a dog license, is identical to sliding a key onto a key ring. Simply pry apart one end using a butter knife, key, your fingernail or something similar. Split O-rings used for canine tags and collars are usually small, so opt for an inanimate object rather than risking bending or chipping your fingernail.
Slide the license over the open end of the ring by lining up the hole at the top of the license with the end of the ring. Keep sliding it along the metal ring until the license slips over the other end. Once that happens, your pup's license is securely attached.
Attach the O-ring to your pup's collar. This is the same idea as attaching it to your pup's license. Open one end of the ring. Insert the ring from your pup's collar -- typically sewn into the collar itself -- into the opening. Continue sliding the sewn-in ring along the split O-ring until the O-ring closes.
Slip the S-hook onto your pal's license by sliding the bottom end of the hook through the open hole of the license. An S-hook, just like an S, has two ends.
Use pliers to clamp shut the bottom end of the hook that holds your pup's license. You want the end touching the curved body of the S so the license can't make a run for it.
Hook the other end of the S over the ring sewn into your pup's collar. Finish the job by clamping it closed with pliers.