After obtaining the parachutes from Leonardo's missions, climb to the top of Castel Sant'Angelo (the castle you visit to free Caterina and again when Cesare kills Rodrigo). You will need to make it to the top of the flagpole, which is relatively easy but time-consuming.
Make sure you are unarmed and have a parachute available, then jump and press Square to release the parachute. The trophy should ding within 10 seconds of releasing the parachute.