Just make sure that every mission you go through, look in the corner how many of the Letters there are, and make sure you find and visit and view them. I tend to just choose harder difficulties so I don't know if difficulty plays into making sure you can find them all or not.
Unfortunately, unlike the caches or bad guy groups, finding one of these does not remove them from the list in countdown fashion. Make sure you keep track in every mission so you don't end up running everywhere again if you want to save time.