Find a zombie spawner in a dungeon
Mine around the spawner to make a room
Light up the area to control other mob spawns
Place torches on the spawner to stop it temporarily while building
Dig a 9×9 room centered on the spawner
Make the room 5 blocks tall
Create water channels to move zombies
Build a drop chute or collection area
Remove the torches from the spawner
Keep the area dark so zombies spawn
Set up a kill chamber for zombie farming
Use hoppers and chests to collect drops