You gain the ability to throw florins during the mission "Arrivederci". First, you must select money throwing as your ability by hitting and selecting it. From there, walk through the streets, and press . Every time you throw money, you will lose 10 florins. You must do this at least 30 times for the achievement.
If you are hard pressed for money, try pickpocketing people first. While you may not get much from most people, it's a steady source of income, and can make this achievement much easier.