Locate the Gerudo Mesa and Cave Of Ordeals in the Gerudo desert to be able to unlock Fairies, and later Great Fairies, at each spring around Hyrule. Successfully complete the indicated number of rooms in the Cave Of Ordeals to unlock the corresponding Fairies. Smaller fairies can be bottled and used to restore up to eight hearts. Great Fairies give you a bottle full of Great Fairy Tears, which fully heal Link and temporarily boost his attack as long as you have an empty bottle and no Tears in your inventory already. Both kinds of Fairies have an unlimited supply once unlocked.

Fairies in the Eldin Spring: Successfully complete 30 rooms in the Cave Of Ordeals.
Fairies in the Faron Spring: Successfully complete 20 rooms in the Cave Of Ordeals.
Fairies in the Lanayru Spring: Successfully complete 40 rooms in the Cave Of Ordeals.
Fairies in the Ordon Spring: Successfully complete 10 rooms in the Cave Of Ordeals.
Great Fairies in all Springs: Successfully complete 50 rooms in the Cave Of Ordeals.