I got a long necklace with the bead arrangement I wanted from Walmart. Ordered the little skulls off Etsy and then went to the jewelry/bead section at Hobby Lobby or Micheal's.
I got some long jewelry pins that have a small loop at one end called eye pins:
. This goes through the skull and can be looped around the existing necklace chain to attach it. (You have to make the loop on the straight end after running it through the skull)
I also got some split rings:
the small hoop on the pin opens and I slip the ring inside,then close it up. This is what the skull "rests" on.
The only tool I used was a set of needle nose pliers from the garage. All the jewelry rings open and close quite easily.
Hope I explained it well enough. If all else fails, take a pic to the hobby shop and ask them how to make it and what you'd need.