You will unlock this achievement after you collect all 16 mayan stones and unlock the door underneath the large temple in Tulam. (Assassin's main place)
Note: Thanks to Da Lukinator for pointing out that you do have to be at a certain point in the game to get some of the stones. I believe the last one you unlock is the one on Long Bay, which you don't unlock until around sequence 10.
Locations of stones:
Eleuthera:
-Cat Island (1)
Dry Tortuga:
-Matanzas (1)
-Cape Bonavista (1)
Conttoyor:
-Tulum (1)
Chinchorro:
-Santanillas (2)
Serranilla:
-Misteriosa (2)
-New Bone (1)
-Isla Providencia (2)
Charlotte:
-Long Bay (1)
Navassa:
None
Punta Guarico:
-Tortuga (1)
Gibara:
-Great Inagua (1)
Castillo de Jagua:
-Pinos Isle (2)
Cruz:
None
After collecting all of these take them to Tulam and place them on the door. Doing this will net you the achievement and get you the Mayan armour. (Deflects metal projectiles)