Instructions
Select the "Summon" command when it's Yuna's turn in battle. This will allow you to choose any of the aeons that you've unlocked at this point of Final Fantasy X. Eight different aeon summons are possible for Yuna to obtain in Final Fantasy X: Valefor, Ifrit, Ixion, Anima, Bahamut, Yojimbo, Shiva and the Magus Sisters. You'll get an aeon with full HP if it's the first time you've summoned that specific aeon.
Find a Save Sphere or an Inn and rest there. Visiting either of these structures in the Final Fantasy X world will heal your entire party, including all aeons.
Fight at least eight battles without using your aeons. Knocked-out aeons will revive fully after a player fights a certain amount of battles. The aeon Valefor will revive after your party fights eight battles. Other aeons take longer; for instance, you must fight 30 battles before the Magus Sisters will revive. Run away from stronger enemies to make reviving your aeons quicker.