The Zanarkand Ruins portion of the game takes place after you have scaled Mt. Gagazet and traversed through its network of caves. Your first visit to the ruins is much earlier on in the game. Head over to the Ruins walkthrough page if you navigated too far forward in the guide by mistake.
The game picks back up at the fire pit which is the same location where the intro scenes to the game occur. Head over to the Save Sphere and save your game before you follow the rest of your team into the Zanarkand Ruins.
Follow the pathway along straight into Zanarkand. The pathway is fairly linear.
There is a treasure chest along the way that contains a 
Fortune Sphere. The treasure chest is in the middle of the pathway so don’t worry about missing it.
There is also a small platform/walkway even further on down the path with another treasure chest which contains a 
Spiritual Targe for Rikku.
The entrance to the Dome has a Save Sphere off to the left side. Save your game before you head in and then make your way towards the entrance.
There is a short cutscene involving a man at the front entrance speaking to Yuna. He will ask Yuna her name and will welcome her and the team to Zanarkand. Proceed into the Dome after this scene.
The team will get to witness some predecessor summoner’s and guardian’s spirits as they proceed in to the next area. There are enemies in this area called Fallen Monks that you can use a 
Phoenix Down on in order to instantly kill them (although they are fairly easy to defeat anyway).
Move forward down the pathway. Make a U-turn once you come out of the other side of the tunnel and proceed along the upper pathway. Use the mini-map to help guide you through the area. There is a treasure chest at the top of the pathway that contains 10,000 gil.
Continue moving forward until you reach the next Save Sphere and save your game (you may see a treasure chest on the upper level but this can be ignored for now). Just past this area is a scene involving Seymour and his mother.
Go down the pathway to the right (eastward) to find a treasure chest that contains a 
Friend Sphere.
When you see the image of Lord Braska, Auron and Jecht pass by make sure you turn around and follow the pathway down to the lower level. There is a treasure chest at the bottom of the path that contains a 
Lv. 3 Key Sphere which is a fairly rare item.
Go through the doorway up ahead and into the next area (listed as the corridor on the menu screen). There is another treasure chest off to the right side that contains a 
Luck Sphere.
Follow the images of Lord Braska, Auron and Jecht up to the top of the stairs. Save your game using the Save Sphere at the top of the stairs and then proceed into the Zanarkand Cloister of Trials.
Before you rush into the next area to confront Yunalesca, there is an upcoming boss fight that requires some preparation. You will not be dealing with the Berserk status effect anymore so you can remove any armor you had on previously with 
Berserk Ward or Berserkproof. Instead, you will be dealing with the Darkness status ailment if your characters perform regular attacks and the Silence status ailment for magic casters.
This means that you should equip any Blind Ward or Blindproof armor you have on your melee characters (Tidus, Auron, Wakka and Kimahri) and Silence Ward or Silenceproof armor on your magic casting characters (Yuna and Lulu).
There is another cinematic in this room where the team will get to watch Lord Braska, Jecht and the younger version of Auron talk about breaking the cycle of Sin. Jecht will reassure Auron and Braska that he “will think of something”.
Proceed in to the next room after the scene is over to confront Yunalesca and begin the next boss fight.
There are a few more lines of dialogue before the next boss battle begins.
There are three phases to the fight against Yunalesca and she will take three different forms during the course of the fight.
In Game Description: Casts Curaga and Regen on zombified characters. Cure zombie with Holy Water. Her counterattacks dispel support magic. She also curses aeons with her Mind Blast.
Phase 1:
HP: 24,000
Yunalesca has two counterattacks that she will use during phase one. If you attack her with a physical attack she will counterattack that character by casting Blind on them. If you attack her with magic she will counterattack by casting Silence on that character.
You have two options: you can use Yuna’s 
Esuna spell to rid yourself of these status ailments or you can use items to remove them (
Eye Drops for Blind, Echo Screens for Silence or Remedy for either). You can also have Rikku use 
Al Bhed Potions to remove the Silence status effect and do some healing at the same time.
Phase 2:
HP: 48,000
Yunalesca will now use a move called Hellbiter during the second phase of the fight. Hellbiter will inflict the Zombie status on all of your party members. Following this, Yunalesca will begin casting curative spells on your party which will inflict damage while the Zombie status effect is active. Your own healing spells and items will also cause damage.
If you remove the Zombie status effect from all of your party members Yunalesca will immediately reuse the Hellbiter spell, so keep the Zombie status effect on at least one character at all times. Have Yuna or another character use a Holy Water on a character if you need to heal them. You can also have Yuna cast Dispel if Yunalesca casts Regen on any of your party members.
Phase 3:
HP: 60,000
Yunalesca can now cast a spell called Mega Death. Mega Death will instantly kill your characters unless they are afflicted with the Zombie status effect. This is another reason why you must keep Zombie active on at least one character at all times.
Additional notes and strategies:
Holy magic is incredibly powerful against Yunalesca and you will likely require Yuna in your party for Dispels and healing. Have Yuna blast away at Yunalesca with this spell (if she has it) to make the fight much easier.
You can easily have Yuna (or Rikku) learn to cast Holy at this point in the game by using a Teleport Sphere or a Return Sphere and a Lv. 3 Key Sphere. Rikku’s standard path goes right by Holy as shown in the screenshot below:
Use Yuna’s Reflect spell on your characters to reflect the status ailments back to her during the first round and to reflect Regen back to her in the second round. Just be aware that you will want to have Yuna cast Dispel on Yunalesca if she casts Regen on herself so that she does not heal herself for too much HP.
Yunalesca will use a move called Punch which will remove all positive status effects with the exception of Reflect, which makes Reflect an incredibly powerful tool during this fight. You will want to cast it on Yuna to make sure that she cannot be silenced during the fight.
Equip your characters with armor that has abilities for negating Silence, Darkness and Confuse, but be cautious about negating the Zombie status effect. You will want to keep Zombie on at least one of your characters.
Steal:
Stamina Tablet (common)
Farplane Wind (rare)
Drops:
Lv. 3 Key Sphere
Overcoming the Past
You will receive the Overcoming the Past Trophy once you have defeated Yunalesca.
Yunalesca will talk to the team one more time before she dies. She will explain that there is no way to summon the Final Aeon without her. She will lament for Zaon as she passes and the team will consider what to do next. You will regain control of Tidus in the center of “The Beyond” room.
Go to the back of the area where you fought Yunalesca and descend the stairs that lead downward at the back of the room.
You will be teleported to the other set of stairs on the opposite side of the room.
You must complete the step listed above in order to make a treasure chest appear at the back of the room near that same set of stairs. Open up the treasure chest to pick up the 
Sun Crest which is used to create Tidus’ Celestial Weapon called 
Caladbolg.
You need to pick up that Sun Crest right now! A Dark Aeon will appear in Yunalesca’s place once you leave this area, specifically Dark Bahamut, which will prevent you from obtaining the Crest until after you are able to defeat it (and it is extremely difficult to defeat).
Do not leave this area without picking up the Sun Crest!
Go back down the stairwell that was used to enter this room for another cutscene. This one involves Auron and Tidus in the Great Hall Dome. Save your game at the Save Sphere and then exit the Zanarkand temple.
Sin will appear seeming to try to communicate with Tidus as you leave the temple. After that, the Airship, Fahrenheit, will approach. You can now make use of the Airship and travel to various locations around Spira.
Note that Final Fantasy X has no overworld like previous Final Fantasy games did. Instead, you enter a destination in to the computer and the Airship will take you there.
