When curing a zombie villager, you will need a golden apple (normal) and a potion of weakness. Splash the zombie villager with the potion of weakness.

As the zombie is under the influence of the weakness potion, you give it the golden apple. You can give it the golden apple by facing the zombie while being about one or two blocks away, then eating the golden apple.

 

This will make the zombie villager consume it and he will start to shake. He will also have red swirls around him as he shakes to show that he is being cured.

 

When he stops shaking, you should get the achievement called “Zombie Doctor”. After the villager is cured, that villager will get a job if it didn’t have one already, then the trades it gives you will be less expensive than before.

Multiple people use this method of infecting and then curing villagers that have very good trades so that they get the exact same trades but less expensive. You can heal the same villager as many times as you like as there is no limit. Use this feature to your advantage and find a villager that had items that you like, and get better deals!