Perform a Leap of Faith from the Top of Florence's Giotto's Campanile.
You can unlock this as early as DNA Sequence 1. In the center of Florence is the Basilica di Santa Maria del Fiore, also known as The Duomo. Standing right next to it is Florence’s highest tower, the Campanile di Giotto. For this achievement, you must climb to the top of this tower, activate the view point and jump from its perch into a cart filled with hay.
To reach the top, you can’t start at the bottom. You must first scale The Duomo and reach the lower roof. From there, stand directly in front of the tower and leap to it. Begin climbing it, and about half way up you must move around to your left to continue to ascend. When you reach an open window, climb in. You will have to continue to climb from the interior. Work your way up to the top, climb on the ledge and stand on the perch to synchronize the view point. To get the achievement, hold down and and press up on the to perform a swan dive into the bale of hay below.
Tip for anyone going for this: You can do this ANYTIME. You do NOT need the climb leap skill. You don't even need the Assassin's clothes. I did it before completing Sequence 1, just because I wanted to reveal as much of the map as possible as soon as I could.