There is some randomness to this achievement, but it's not too bad.
To earn this achievement you will need to read only the 25th journal entry.
The entries are found in the following manner:
Entry 1: Found in the foyer when you enter the castle. Can't really be missed. Just remember to read it.
Entries 2 - 24: Randomly appear through the rooms/areas. If you are playing as the Spelunker, they might be one of the ? rooms indicated on the map (could also be a mini-boss fight among a few other possibilities). The rooms are always a single screen worth of area if there is a journal entry. They can occur in any of the four zones of the map. This is why it makes sense to fully explore areas even after you have defeated the boss for that area.
Entry 25 - This entry is found outside your final fight with Johannes. This area is locked until you have defeated all 4 of the area bosses.
NOTE/EDIT: As mentioned by xDartz xNVx, you technically only need Entry 25 for the achievement to unlock. So you could skip all the others and just remember to read this one right before the fight with Johannes. I personally enjoyed reading them as it gives a little backstory to everything, but it is not required.
EDIT #2: As it stands right now, it is possible to earn this achievement without actually reaching the final boss. There is a glitch currently where if you read the other 24 entries and then read a journal, the achievement may pop without #25. I cannot verify this, but numerous comments have pointed out this works.