Pick a dark area far from caves, villages, and other spawnable spaces
Build a large flat spawning platform high above the ground or over the ocean
Use solid blocks for the platform
Add walls around the edges to keep mobs from falling off
Place trapdoors, buttons, or slabs to help mobs path toward the center or edges as needed
Create a central drop chute or water channel to move mobs downward
Add water streams to push mobs into the chute
Make the drop tall enough to weaken mobs but not kill them if you want manual collection
Place hoppers at the bottom to collect drops automatically
Connect hoppers to chests for storage
Keep the area around the farm lit or spawn-proofed so mobs only spawn in the farm
AFK near the farm at the correct distance so mobs spawn efficiently
Test the farm and adjust lighting, spawning platforms, and mob movement if needed