First off, you need to collect 5 singing crickets from graves in graveyards you visit throughout the game. This unlocks the song.
Equip the song by going to the gear menu and then go down to the last option (the one with an image of a Horse) and then select the song from there.
Then after the "Reckoning in blood" main quest you can play the song at
by swiping left on the touchpad to gain the trophy.