There are 3 missions in total. The first two are in New Orleans and you must be in the lady persona for them to appear on the map and the last is found in the Bayou wearing the Assassin Persona. (On the map press Y to see the icon for "Free slaves" mission if you have trouble seeing them).
Completing all three short missions nets you the achievement.
Spoilers: More detailed mission parameters/locations below
The first mission "A Family Divided" is located at your family mansion.
(It is to the left and slightly up from the church aka the grey building in the center of the map).
You will have to find a slave, take out the two guards with him and escort him back to your mansion.
Optional: Take no Damage (Not needed for achievement)
The second mission "Earthly belongings" is located at the docks just south of your headquarters.
You have to help the man retrieve his belongings, he will lead the way to them. Simply find and loot the chest and return the items to him. (Remember the Lady Persona can bribe guards to let her pass by).
Optional: Kill no one. (Not needed for achievement)
The Third mission "A New Recruit" is found in the south of the small village (in the north part of the map) that you start in upon entering the Bayou.
You have to go find the mans wife who he fears has joined the "rough men". Go to the destination and kill the three guards to finish the mission and unlock the achievement.