When you clear the game, check the Notepad section on your Gamepad and you'll find a note that wasn't available previously. It consists of a 4x4 grid, with the numbers 1 through 5 placed throughout it. You can press + on your Gamepad to access the Encyclopedia page. Scroll down and you'll find a cipher on the page's lower half, which corresponds to the grid you saw in the new note. You can cross-reference the two resources to enter the correct symbols on that page. When you correctly match the symbols to their numbers and enter the results, you'll unlock the journal.