There are two ways to get to the DLC area for Dark Souls 3: The Ringed City.
If you have completed Ashes of Ariandel, a bonfire will appear in the back of the Father Ariandel boss room if you have beaten him and Sister Friede, and have talked to The Painter in the attic of the chapel.
If you have not completed Ashes of Ariandel or do not have Ashes of Ariandel, a bonfire will appear at the final bonfire at the Kiln of the First Flame before the Soul of Cinder boss fight.
Good luck!
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