This quest will be unlocked after completing the quest "This Old Cove". To start it, you must sail to the western edge of the map, where the ruins of Tulum are located, and drop anchor at the designated spot.
Finding himself in a village crawling with men and woman who would not appreciate his presence, Edward sneaks through an Assassin village to locate Master James Kidd.
After going ashore, stick to the right side, climb the wooden scaffolding, and walk along the rocks - then hide in the tall grass and keep moving forward, still staying to the right. This way, you'll get past all the soldiers on the beach and reach a path that first leads slightly uphill and then further inland.
Another, slightly more open area is guarded by five guards. You can kill them - which won't be difficult thanks to the tall grass - and sneak past on the left. You can also take the path on the right and climb a tree. In that case, you won't have to kill anyone.
The next area has four soldiers, one of whom is a marksman. You can sneak past them or kill them; your goal is to sneak behind the stone pillars on the left side, where you will find the path leading further.
The path down is short and patrolled by a soldier you can't get past, so you'll have to take him out. On the other side of the path is a clearing with a waterfall and a small pond. Eliminate the two soldiers at the bottom without being spotted by the marksman standing on the rocks above the waterfall - then climb the rocks on the left side and head up the path.
Head forward and eliminate the next two soldiers in your way - shortly after, you will find yourself behind the marksman. Approach him and assassinate him.
You are close now - you still need to make your way along the left side to the other side. Two more soldiers will stand in your way, including a Brute.
When you reach the other side, you will see an assassin kill two soldiers. You can safely approach him, and he will lead you to Kidd - who is actually standing just a few steps away at the entrance to the temple. To complete this quest, talk to Kidd. A cutscene will trigger in which you will meet Ah Tabai, the Assassin mentor - and once you finish talking, you and Kidd will enter the temple, where the next quest will begin - The Sage's Buried Secret.
