To unlock the Lend Me Your Ears trophy in Days Gone, you will need to collect a total of 989 Freaker ears. Whenever you kill a Freaker, it will automatically drop and ear which you can collect be simply walking over it.
Once you complete the story, all Hordes will automatically be marked on your map, with bigger Hordes containing 100+ Freakers found in the 5th and 6th regions of the map. Taking out a few Hordes when grinding for the final skill points you need for the platinum will give you massive progress if you haven't unlocked the trophy already by that point.