Light the sacred flames at all three desert shrines.
It's hard to miss the quest start for this, it is located at the very start of the DLC.

The first one is located in the very first desert area, near the entrance to the giant worm.
The second one is located almost right at the start of the crimson flats area, simply go north.
The third is at the very end of the area, near the temple of Azunai.