There are several ways to find pieces to treasure maps, buy them from people in a town’s tavern, accept and carry out a mission for the women in the tavern or find them scattered around the Caribbean yourself. Each treasure map has 9 parts and you will receive 1 part per mission. As more of the map is revealed you will be able to tell where on the nautical map the treasure is hidden. Once you know where the treasure is hidden, set sail and be sure to move your ship directly over the location marked on the map. As soon as your ship moves over the treasure’s location the achievement will unlock.
TIP: You don’t need all 9 parts of a map in order to locate the treasure but it seems you need at least 5, as the fifth part will reveal the treasure’s exact location.