Gather building blocks, slabs, water buckets, hoppers, chests, trapdoors, and a killing method
Find a dark area high above the ground or build over an ocean to reduce nearby mob spawns
Build a large enclosed spawning room with a dark interior
Make the room mob-friendly with solid floors and a 2-block-high interior space
Add water channels to push mobs toward a central drop or collection point
Place trapdoors or similar blocks to help mobs fall into the drop area
Build a drop shaft that lowers mobs to a killing chamber
Put hoppers and chests at the bottom to collect drops
Add a kill method such as fall damage, magma blocks, campfires, lava, or a player kill chamber
Light up all nearby caves, surfaces, and structures to improve spawn rates
AFK near the grinder so mobs spawn inside it instead of elsewhere
Test the grinder and adjust water flow, spawning space, and kill chamber as needed