Minecraft wiki entry for Mob_farm, associated text: 1 dispenser (per layer)
1 water bucket (per layer)
1 observer (per layer)
112 opaque blocks of choice (per layer)
1 clock (e.g. an Ethonian hopper clock: 2 hoppers, 2 sticky pistons, 2 comparators)
A capture layer (just several dozen hoppers leading into a chest)