So, you're curious about turning a friendly villager into a groaning, shuffling zombie villager? Alright, let's talk zombie transformations (within the safe confines of your Minecraft world, of course!).
The process is fairly straightforward, but risky for the villager. You'll need a zombie and a target villager. The zombie simply needs to attack the villager. On Normal difficulty, there's a 50% chance the villager will become zombified upon death. On Hard difficulty, it's guaranteed! On Easy difficulty, the villager will just die (sorry!).
Once zombified, you'll have a Zombie Villager! But the fun doesn't stop there. You can actually cure a Zombie Villager back into a normal Villager using a Potion of Weakness (splash potion is easiest) and a Golden Apple. This cured villager will then offer you incredible trades! So, while turning a villager into a zombie might seem cruel, it's actually a pathway to discounted emeralds. Good luck, and happy (undead) villaging!