You have to purchase 8 Brigmore Witches favors.... out of 10. Two, however, depend on how you played in the previous DLC (The Knife of Dunwall) - thanks Euler13 for pointing this out. So there might only be 8 favors available for you. In that case you need to get ALL of them.
These are available for everyone:
Coldridge Prison (A Stay of Execution for Lizzy) - 3 favors (400 coins)
Draper's Ward (The Dead Eeels)- 2 favors* (300 coins)
Brigmore Manor (Delilah's Masterwork) - 3 favors (325 coins)
You can only purchase favors at the very beginning of a mission at the mission loadout screen. Make sure you have enough money to buy them! Do not leave the loadout screen without buying them!
* Euler13 adds: There are two more favours available in the Dead Eels area depending on the decisions you made in The Knife of Dunwall. If you completed "Free the Captive Laborers" during the first mission then there will be an extra favour in the Dead Eels area (75 coins). And there will be another karma related favour in the Dead Eels area (100 coins) depending on how you "helped" Ames in the slaughterhouse.