Heal a zombie villager - how?

Do it somewhere with a roof. It's still a zombie for the whole time it's shaking, so if the sun comes up it burns and you lose the golden apple with it. Trapping it in a hole with a block over the top works fine, and a name tag stops it despawning if you have to leave it a while.

splash potion of weakness on it, then feed it a golden apple - the plain one, eight gold around an apple. enchanted adds nothing here. it shakes and makes an awful noise for a few minutes and then you have a villager. the reason it's worth doing at all is the prices afterwards, a cured one trades absurdly cheap and it stays that way.