Hey Guys =)
I bought Kingdom Hearts 2 Final Mix for PS3, and I decided to make a guide while I play the game, with videos in this guide =) Have fun!
(On going...)
Puzzle Pieces
(The pictures are in history's game order from top to bottom)
-Every 2nd world keyhole is unlocked after Demyx battle, when Mickey enters into the darkness portal
Hollow Bastion / Radiant Garden
=>I got it first =p
After locking the Twilight Town & complete another world 1st boss, you can talk to Uncle Scrooge. Click on the Image to watch the Video <= 
=> Merlin's house, only available after the first scene with Leon and Organization XIII.
=> After unlocking Twilight Town
After helping Leon group fighting the heartless<=
=> After defeating MCP from Space Paranoids.
Beast's Castle (9 Pieces)
=> After defeating the Thresholder
=> After helping Beast getting to his senses. After defeating Xaldin, you have access to the bridge.<=
The Land of the Dragons (10 Pieces)
=> By mistake I took it again =(
=> You can use the vortex nearby to catch it.
100 AcreWood (6 Pieces)
You get Piglet's House page in the King's Room at Disney Castle <=
=> Hollow Bastion's Crystal Fissure.
The Land of Dragon's King Throne room <=
=> Agrabah's Ruined Chamber.
Pride Land's Oasis <=
Olympus Coliseum (9 Pieces)
* => 2 on a different platform.
Disney Castle (5 Pieces)
Timeless River (3 Pieces)
Atlantica (3 Pieces)
Port Royal (13 Pieces)
=> 2 in the city
You get this next 3 in Jack's Black Pearl <=
=> Obtained after 2nd keyhole.
Agrabah (14 Pieces)
*
Use magic to destroy the market selling posts * <=
=>3 in Chasm of Challenges. <=
=> 2nd Keyhole
Twilight Town (20 Pieces)
(1st one you get after Sora changes his clothes at the beginning of the story)
=> After Kairi coming to this town
=> 2nd Twilight 
Space Paranoids (4 Pieces)
=> 2nd Keyhole
Halloween Town (8 Pieces)
=> You can get here after
defeating Oogie Boogie.
Pride Lands (10 Pieces)
The World That Never Was
Magic Spells:
(Number* + Skill Name)
1/Blizzard - Talking with Merlin at the first Hollow Bastion arrival;
-7/Blizzara - Defeat Demyx during the second visit to Hollow Bastion;
--17/Blizagga - Clearing all five parts of Atlantica;
2/Fire - Defending the gates at Hollow Bastion with Leon;
-13/Fira - Is obtained after clearing the first visit to the Pride Lands;
--12/Firaga - Defeats Jafar during the second visit to Agrabah;
-9/Thundara - Defeating the Storm Rider in The Land of Dragons;
--15/Thundaga - Defeating the Groundshaker during the second trip to the Pride Lands;
3/Cure - Complete the first visit to Beast’s Castle;
-8/Cura - After defeating Demyx, when Goofy joins up the team again;
--14/Curaga - After finding all of the Torn Pages 100 Acrewood;
6/Magnet - Defeat Oogie Boogie during the first visit to Halloween Town;
-11/Magnera - Defeating the Grim Reaper for the 2nd visit to Port Royal;
--18/Magnega - Defeat Xigbar in The World That Never Was;
5/Reflect - Defeat Pete in Timeless River;
-10/Refletera - Defeat Xaldin during the second visit to Beast’s Castle;
--16/Refletaga - Defeat MCP during the second visit to Space Paranoids.
*order of spell adquire in game history.