Minecraft wiki entry for Drowned_farming, associated text: Make the ceiling completely from glass blocks, with at least six blocks between the floor and bottom of the glass. 1 block of stone wall (excluding the slabs on the top edge of the wall) should extend above the top of the glass, to form a shallow pool that you must fill completely with water source blocks (no flowing water). You can build a temporary infinite water source off to the side to work with as you fill the pool.