Use your bow if you’re using Weak Arrow (don’t enchant your bow with Strength or Flame, otherwise it can kill a zombie villager), or lure a witch near a zombie villager or vice versa. Because the zombie villagers are impervious to poison and because they are a bunch of the undead, the witch’s potions, which save from weak and slow spawners, have no effect on them. Therefore, you should not worry that they will attack the witch after the witch throws the weak potion. read more how to cure a zombie villager
Toss splatters/stretch potions at zombie villagers
Toss splatters/stretch potions at zombie villagers or shoot slanted arrows from a distance. Instead, wait until the witch pours a potion on the weak spot while you’re within three blocks of the witch and zombie villagers. If done correctly, there will be gray swirls coming from zombie villagers; this indicates that it is currently weak, as it is a horde of undead, giving it life (giving “life energy” is arguably the first part of the cure).
Give the golden apple to the zombie villager by facing him and pressing the use key. If done correctly, the zombie villagers will begin to shake and emit crimson particles. Remember that the bed and iron rod accelerate the curing process. At this point keep the zombie villager safe and don’t attack it.