Here is the world map:


And here is the map with the location of the NPC you need to talk to in order to unlock the Light ending:



Except it's missing one step I'll detail in the next section.

First of all, you need to unlock the sequence by finding the NPC. To do so, warp to the silent altar, go left two screens and drop to land on the elevator. Hit the wall on the right to reveal a hidden area. In that area is Sarah, the NPC you need to talk to to start the sequence unlocking the Light ending.



From there, teleport to the ruined danche and proceed to the DLC area. On your way, you should meet her a first time. Then it's pretty straight forward, make your way through the DLC and talk to her on the areas 2, 3 and 4 marked on the map.

Then go beat the final boss. This time, don't go right and don't use the relic but instead go left. You'll have a new option available that will take you to Sarah. Accept and you'll get the Light ending!

Thanks to Gustav, the dev of the game, for providing the maps you can find in this guide.