Another trophy that is more of a time sink than actually challenging, but odds are you will unlock this naturally in your playthrough.
Codex entries come from most items you can Read or Examine in the world. This includes the first time you pick up a new type of weapon, herb, crafting material, or even the people you meet in the world. Anytime you interact with items of this nature you will see on the left hand side of your screen an entry for "Codex Unlocked".
While 250 entries sounds like a lot there are WAY more than that provided you don't mind looking for them. I myself unlocked this trophy only 22 hours into the game just after reaching Skyhold for the first time.