Yoel is located under the first bonfire of the Undead Settlement, in a dead end with a bunch of other pilgrims. Speak with him to accept his service and he'll move to Firelink Shrine, at the dead end to the left of Andre the Blacksmith, where you can speak with him to obtain the "Beckon" gesture and purchase Sorceries.
Yoel will offer to "Draw Out True Strength" in exchange for a Dark Sigil , which causes hallowing upon death - decreasing your max HP each time you die, equal to the number of Dark Sigils you have.
WARNING: Healing the Dark Sigils will void the "Usurpation of Fire" Ending''
After obtaining 5 Dark Sigils:
After obtaining five free levels via the "Draw Out True Strength" option, Yoel will appear dead in Firelink Shrine and Yuria will appear.
WARNING: This must be done before defeating the Abyss Watchers, as he'll die at this point no matter how many Dark Sigils you have obtained and possibly ruining the ending.
- Hollow's Ashes