For this you need to do EVERYTHING in the DLC, not just get 100% in the main missions like in the base game.
Your total sync needs to be at 100%... Meaning :
- Complete all 10 story missions with every optional objectives done.
- Complete all 21 loyalty missions, with every optional objectives done.
- Grab every collectibles (chest, locked chest, posters and helix bugs) in London (Whitechapel, City) as well as the 3 "mission maps" (Manor mission 4, Deptford mission 6 and Asylum mission 10).
For the loyalty missions, you'll need to progress to story mission 6 for the last activities to become available.
For the collectibles you can purchase maps at any store in the game for 5000 each.
Time consuming but not very hard.