Although there are already hundreds of similar tutorials on the internet, I will still write a mini-guide about one of my favorite game endings

Find Yoel of Londor near the Foot of the High Wall bonfire. Accept his services, and he will go to the Firelink Shrine.

Yoel will increase your character level in exchange for a dark sigil that increases your character's hollowing level after each death. You can get 5 free levels in total.
After gaining five levels, Yoel dies, and then Yuria of Londor will appear in the Firelink Shrine.
Important: you need to talk to Yoel and get the levels and get Yuria to shrine before killing Abyss Watchers.

Next, you need to do a few things for Anri's quest:
Find Anri of Astora at the Halfway Fortress bonfire.
Speak to her again at the Firelink Shrine after killing the Deacons of the Deep.
Find Anri in the Catacombs of Carthus in a hallway next to the first rolling skeleton ball:
Choose dialogue option "no".
Then enter the Smouldering Lake and kill Horace in the cave (hug the right wall when entering the lake).
Find Anri on the path near the wooden bridge before the High Lord Wolnir boss and choose dialogue option "Tell Anri of Smouldering Lake"
Find Anri at the Church of Yorksha fire. Exhaust all dialogue.
After killing Pontiff Sulyvahn, speak to Yuria at the Firelink Shrine. Exhaust all dialogue options using talk.
Perform the "wedding" ceremony in the Darkmoon tomb (located behind a false statue in the room before the rotating tower to Anor Londo): talk to the Pilgrim NPC and accept the sword. Use the sword on your spouse and receive 3 dark sigils.

After killing Soul of Cinder touch the fire and enjoy the ending