Sorceress Sellen's questline begins as an early sorcery connection and later expands into a longer story involving primeval sorcerers, Witchbane Ruins and Raya Lucaria Academy.
Waypoint Ruins
Meeting Sellen at Waypoint Ruins establishes the questline and her role as a sorcery teacher. Keep this early relationship separate from the later body-transfer and Academy steps.
Primeval sorcerers
The middle of the route connects to powerful sorcerers found elsewhere in the world. Record which one you have already located so the quest does not become a vague “find another sorcerer” task.
Witchbane Ruins
Later steps involve Sellen's situation at Witchbane Ruins and progression that can move the storyline toward a new location.
Academy finale
The quest eventually returns to Raya Lucaria and presents a meaningful late choice. Treat that decision deliberately because the route and rewards differ.
Why this quest is easy to lose
It spans several regions and changes context dramatically. A one-line note such as “Waypoint → primeval sorcerers → Witchbane → Academy” is enough to preserve the structure.
Reference: Elden Ring Wiki — Sorceress Sellen.