Reach the Subterranean Shunning-Grounds beneath Leyndell
Find and defeat Mohg, the Omen
Open the hidden passage behind the altar in the Cathedral of the Forsaken
Drop down the platforming path to the Frenzied Flame Proscription
Remove all armor
Open the door by interacting with it while completely naked
Meet the Three Fingers and accept the Frenzied Flame
Finish the game normally after defeating the final boss
Choose the Frenzied Flame ending at the end of the game