After starting the questline by speaking to Anais, complete the introductory quest, 'Where We Left Off' and then speak to her again to begin 'Runaway'. After completing this, speak again to Anais to begin 'Unplanned Parenthood' and complete that. Finally after completing that, speak to her one final time to begin 'A Night in Tegea', completing which will complete the DLC. There are choices you can make during these quests that will affect the outcome, however they are irrelevent in regards to unlocking the achievement.
This is thanks to the user "Scotty" from xboxachievements.com