You unlock new codex entries by learning new information about the Dragon Age world, most of which will become available for you to read at any time through the appropriate menu. You will earn new codex info the first time that you:
- Talk and explore unique conversation options with people;
- Gather plants, minerals and create new unique consumable items;
- Kill wildlife and different enemy types in general;
- Find and interact with certain objects, such as reading wall signs with the
button.
You don't have to play the whole game focusing on finding them (the game has more than just 250 pieces of codex), but it helps to unlock the achievement sooner if you take advantage of all the early opportunities.
If your playstyle is as thorough as mine, you will be able to unlock this achievement on your first visit to Val Royeaux. That means I had only explored 5 out of the 20 areas of the game prior to that point.
Note that when I did that, I've intentionally beaten all available missions in those 5 areas before trying to progress with the Main Story. In other words, I had finished and explored the maps as much as possible in the following locations:
- Haven (explored the whole map and talked to everybody there is to talk to).
- Hinterlands (you won't be able to enter Redcliff yet and except for the upper right corner of the map where there is a High Dragon and which I decided to avoid since I'm playing on Nightmare difficulty, I've done all the other quests).
- Fallow Mire (did everything there is to do there, explored the whole map).
- Storm Coast (explored the whole map in my first time there, except for all that is behind the sealed door).
- Forbidden Oasis (explored as much as I could and just didn't get through all of the doors at the temple because I didn't have enough shards).
If this info helps you in any way towards the achievement, you're welcome to let me know it by leaving a thumbs up and a comment.