How do you get the codex on assassins creed 2

there are 30 codex pages in the game and you will have to find them around the cities to find them on the map you have to go up to view points that you have located and sync if there are any codex pages are in range they will show up on the map and they are always have 4 guards posted around a open door (if you don't want to fight them use thieves, contessas or mercenaries to distract them while you loot the box inside)