If you have a Fire Keeper Soul, you can give it to any of the Keepers and they will increase your Estus Flask strength by +1, up to a maximum of +7.
Once you have reached +7 on the Estus Flask, you can consume a Fire Keeper Soul in return for +5 Humanity and full health.
Technically, you can consume the Fire Keeper Souls at any time, but it is not worth doing this until you are at +7 on your Estus Flask.
You can carry one of each of the 7 unique Fire Keeper Souls in your Bottomless Box and one of each in your inventory for a total of 14 Souls in total.
A few of the descriptions make it clear which Soul you are carrying, but a few of them have a generic description.
Anastacia of Astora in Firelink Shrine. A promotional screenshot as she appears in game below the Bonfire.
- On the Altar in Undead Parish
- Near the start in New Londo Ruins. After the wooden walkway, look back towards the lift to Firelink Shrine.
- Near the Blighttown waterwheel lift. Heading upwards, after the lift, go past one of the ladders and into an area guarded by Blowdart Snipers and Flaming Attack Dogs.
- In the lowest, large cell in the The Duke's Archives. You will need the Archive Tower Giant Cell Key first.
- Allow Knight Lautrec of Carim to kill Anastacia of Astora. It is best to return this one.
- Kill the Fair Lady or Lady of the Darkling. Both will drop a Fire Keeper Soul, but their Bonfires will go out.