To obtain this achievement you need to complete the in-game achievement "Honor Among Thieves." You can find this in your pause menu under Journal > Achievements > DLC > Thieves Guild.
You're going to need to advance in the Thieves Guild skill line to level 10 to unlock the last side quest needed for this achievement. But if you're going through the story naturally, you should be already around Thieves Guild level 8 by the time you finish the main questline (Prison Break).
From my experience, it seemed that a new main or side quest for the guild opened up every time you leveled up the Thieves Guild skill line in your skills. To level up the Thieves Guild skill line you're going to have do repeatable missions found in the Thieves Den in Abah's Landing. They're easily identified by the light blue quest markers on your compass.
Each of the 4 main members of the Thieves Guild has a 'personal matter' that you take on in the form of a side quest. The side quests you have to finish to unlock this achievement follow:
- "A Life Deferred" (Quen)
- "Seeds Untilled" (Velsa)
- "What Price, Freedom?" (Walks-Softly)
- "Loose Ends" (Zeira)
Lastly, to start these quests you'll find notes from your guildmates in the Thieves Den with black quest markers over them. Simply read the note and begin your quests.