You'll be able to accept this Stranger Mission, once you've completed the The Joys of Civilization Main Mission in Chapter 4. Head over and enter the Saloon that's located to the north of Saint Denis, where you'll meet a French painter named Charles Châtenay inside.
The Artist's Way Part II
Once a few days have passed in the game, you'll be able to find that Charles has gotten himself into a bit of trouble in an alleyway, simply threaten the man to get him to walk off. Now that you've saved Charles he'll invite you to his Art Expedition.
The Artist's Way Part III
After a few more days have passed in the game, head to Charles' Art Expedition and have a look around at what he has got to offer. Eventually you'll find yourself in a fist fight, once you've knocked the guy out follow Charles Châtenay across the street to hide for a while.
The Artist's Way Part IV
You'll need to wait a couple more days in game before being able to meet Charles again which you'll notice is now dressed up as a lady to aid his escape. You'll need to help Charles get to the port, but this will require you to be a little sneaky as there are people looking for you.
You'll need to use the tram to allow you to hide from the first couple of enemies, and then you need to head through the Post Office to stay out of the way of some more enemies that are across the street, once you reach the port you'll need to have a quick fist fight to take out some enemies and complete the mission.