Find a herald to reduce your notoriety and bribe him for 500 florins. Once done, quickly steal from him. You will get the 500 florins back, plus 3 to 5 extra florins. Repeat this one more time, either with the same or different herald, and the trophy will unlock in seconds.
Go to Florence and look for a big group of people.Your goal should be to have those people walking towards you. Hold A and walk through this group of oncoming people while pickpocketing them. Since that button also serves as the "fast-walk", you can quickly get florins from citizens if you keep it held.