Setup - Before entering the game make sure you have the In Plain Sight gobblegum equipped.
Bow - Fill up all 3 dragon locations. Get the Wrath of the Ancients bow.
Gobblegum - Find a gobblegum machine and do it until you get In Plain Sight if you haven't already.
Skulls - There will be three skulls you have to shoot with the bow. You have to activate In Plain Sight to see the skulls. Also, all the skulls will be floating. The first one is at the ledge of the castle by Mule Kick. The second is in front of the church, in front of the stained glass. The final is right outside of the Double Tap machine.
Skeletons - Now every zombie should be a skeleton. This will last for the duration of the game.
Zombie - if you want to change the skeletons back to zombies, then go to the interactable dragon statue and go to the fireplace and interact with the 3 skulls on the chair.