Minecraft wiki entry for Iron_golem_farming, associated text: Construction is a matter of building the sleeping bunker with two floors (and a stairway between them) with 10 beds on each floor, at least one doorway to at least one bridge that rises up high enough to fit an iron golem underneath (the screenshot shows a bunker with two bridges for easier pathfinding), and making 7-8 blocks of spawning area all around the bunker (starting 1 block higher than the lowest floor of the bunker). The rest is just water management, building channels to guide the iron golems into a killing area, which requires a block of lava, a hopper, and a collection chest.