Posted on 10 November 19 at 22:42
Considering your post was written over a year ago, I'm assuming you got your answer and completed it already, but for posterity:
(Taken from the walkthrough)
There are two ways to start this DLC. The first is to speak to an NPC named Oswald, who can be found in the Grand Chapterhouse in Stonebridge City. The second is from the Causeway Hub after defeating Jeyne.
For this DLC walkthrough I will be assuming that you are starting it after completing the main game. So from the Main Menu, select “Load Game”. As you scroll down the list you will see an Endgame Autosave for each of the characters.