Minecraft wiki entry for Drowned_farming, associated text: A location, preferably in deep ocean, preferably at least 50 blocks away from land
A tower to climb, from which to build structures at high altitudes
A killing platform
A drop shaft
A drop platform; a small water platform some distance (like 20 blocks) above the killing platform to funnel drowned to fall into the drop shaft and onto the killing platform
A spawning platform at least 24 blocks above the killing platform