Simply find a horse and donkey, tame them and group them together in fences or walls. Feed them a golden carrot (8 gold nuggets around a carrot) or a golden apple (8 gold nuggets around an apple) and let the magic happen! It took over a minute once the mule was born for the achievement to pop.
Edit: this unlocked on a brand new world for myself and wasn't able to activate love mode on imported worlds. There are messages conflicting so if your imported world doesn't work, try the seed from the cartographer achievement guide as it spawns you next to three villages and on my version, had a donkey and a horse near the first village.