Courtesans are the groups of women scattered about the cities with a blue flower icon above their heads. You gain the ability to hire them during the memory ‘Fitting In’ in DNA Sequence 2 after you train with Paola how to blend in with people.
When you hire Courtesans, they follow you around to distract any guards that succumb to their charms. To hire them, stand near them and press . It costs 150ƒ each time to hire them. As a result, you will have to hire Courtesans 34 times for this achievement to unlock. You can keep walking around finding them or just play through the game and use them only when needed.