Minecraft wiki entry for Advancement_guide, associated text: Throw a splash potion of Weakness at a zombie villager and give it a golden apple (by facing the zombie and pressing the use key with a golden apple in your hand). Then, wait a few minutes. Don't let the zombie burn in the daylight. You can do this by having something blocking the zombie overhead, giving it Fire Resistance, or a helmet to keep the zombie from burning. Alternatively, you can do this at night.