There is a really easy way to get this achievement in no time.
The easiest way to do this is in New Orleans. Make sure you are using the Assassin outfit.
Now go to the middle of the city and kill enough guards to completely fill up your Wanted level. What you now want to do is to find a magistrate (they have a black + white silhouette symbol on your map). These are the guys that you can pay to get rid of your Wanted level.
So go to one of them and pay him to lower your Wanted level (it will cost you around 1'000 écu). Now immediately hold the B button to steal back your money.
Repeat this 4 more times and you are done!