The Circuit Playground Express already comes with everything we need except an on/off switch. If you're making this project with kids and don't want to do any soldering, you can skip this part and just plug the battery directly into the circuit playground. You'll turn the necklace on and off by unplugging the battery.
I've found that having a switch makes the necklace much more user-friendly, and makes your battery last longer since you can turn it off when nobody is looking.
Cut both ends off your battery cable and trim off one of the outside legs on your on/off switch.
Solder the black wires of the battery cable back together (it's just shorter now). Solder one red wire to each of the two remaining legs on the on/off switch. You can solder either wire to either leg. The way the switch works is that when the button is centered between the two connected legs, it connects them and power flows. When it's centered above just one leg, power does not flow.