In "Assassin's Creed 2," you take on the role of Ezio Auditore da Firenze as he seeks out and kills the men who murdered his family. On this quest, he becomes embroiled in the ancient battle between the Templars and the Assassins, a battle that was first started by his ancestor Altaïr Ibn-La'Ahad. In order to defeat the Templars, Ezio must find the scattered pieces of Altaïr's diary, called a Codex, then solve the puzzle Altaïr left within the Codex.
Locate all of the Codex's pages. There are Codex pages in every city in Assassin's Creed. By pressing the "Select" button on your controller, you will bring up the map of the city you are currently in. Every Codex page in your current city will be clearly marked on the Map.
Acquire all of the Codex pages. Go to the map markers where the Codex pages are located. There will be four guards outside of every Codex location. Kill them, enter the small building they were guarding, then open the treasure chest inside. The Codex page is in the chest.
Take the Codex pages to Leonardo da Vinci. The Codex pages are written in a cypher that only Ezio's friend Leonardo can decipher. As he breaks the code, Leonardo will grant Ezio new equipment and raise his health.
Return to Ezio's villa at Monteriggioni. Go to Mario's office at the back of the villa and interact with the "Codex Wall" to place the Codex in its frame on the wall.
Get the Apple Of Eden. This is a main storyline event, and you cannot avoid doing it.
Solve the puzzle. After you get the Apple of Eden, Ezio will return to the Villa and use it on the Codex Wall. You will then be prompted to use Ezio's "Eagle Sight" ability, which will now display the puzzle. You will now have the option of moving and rotating the Codex pages to try to form a picture. The picture is actually a map of the world done in red lines. It is helpful to arrange the edge pieces first. The map is supposed to be enclosed in a square box, which means every edge piece will have a line segment on it. By connecting these lines you will construct the box, making arranging the rest of the pieces far simpler.