Things you'll need:
RGB color changing LED
Maybe a Soldering Iron
Possibly some fishing string
Step 1: Put It Together
Cut a round hole in the back of the plastic skull.
Step 2: The Setup
Tape the LED to the Cell battery with the correct poles touching.
Step 3: You're Done
Now tape the battery and LED into the hole on the plastic skull.
Step 4: Optional Things
You can solder the LED terminals to the battery or even use and other color of LED if you prefer a solid color. Even add a string to make it a necklace.