While you're playing through the levels in the game, you'll notice that both of your Reveal and Delete letter (letter aids) counters increase the further you progress in the game. I highly recommend
waiting to you have a combined amount of 50 letter aids then use all 50 of them. Once you get into Level 5 you should have enough letter aids to unlock the 'Prepared'
achievement. You can't use all of them at once, you can only use a certain amount on each trivia picture. So use as many as the game will allow you on each trivia picture, then move on to the next one.
While using all 50 letter aids, you should also unlock the other achievements for using letter aids too: