Minecraft wiki entry for Superflat_survival, associated text: You need to kill enough zombies for 10 iron ingots. With the iron, craft a cauldron and a bucket. Place the cauldron down and allow it to fill with water from the rain. Once the cauldron is full, use your bucket to dump out the water in a hole. If you killed any witches, you can use their dropped glass bottles (and potion, drink that to get glass bottles) to collect water, refill the cauldron and quickly make a second water source block. With an infinite water source, you can work on a zombie -> drowned converter.