I thought I'd make a guide as I wasn't sure how to do it at first. If you press the Menu Button when on the main screen, it brings up a little menu, and search is the first option which brings up a search bar with the letters to type.
You can obviously search for anything, but the easiest way is to just press a letter, such as A, then when the list of results come up, select one of them. The first option that comes up if you search for "A" is "a". As soon as you select one of the results it counts towards the achievement, even if it brings up no results... It can be the same search every time and it still counts.
Then all you have to do is repeat this 25 times. The easiest way to repeat it is to press down on the thumbstick which selects the "&12", then move right and press A again, then select the result that comes up. You don't have to delete the letter(s) already there.
Hopefully this helps anyone unsure how to get this.