Open the Editor. Go to the Items tab. Make a spawn table. Add tiers to the spawn table. Add items to the tiers. Modify spawn chance values. Place spawns.
Open the Editor. Go to the Zombies tab. Make a spawn table. Add the ID of an item spawn table to it. Add items to the different clothing groups available on the spawn table. Modify clothing spawn chance values. Modify stats to your liking. Choose whether or not it is a Mega Zombie spawn. Place spawns.