Ranni's questline is one of Elden Ring's longest NPC routes. It crosses several regions and is closely tied to Nokron, Ainsel River, the Lake of Rot and Moonlight Altar.
Checkpoint 1: Ranni's Rise
After reaching the Three Sisters area beyond Caria Manor, speak with Ranni at Ranni's Rise and accept her service. Talk to the characters connected to her household before continuing.
Checkpoint 2: Radahn and Nokron
Progression toward Nokron is tied to defeating Starscourge Radahn. Afterward, explore Nokron and obtain the Fingerslayer Blade, then return it to Ranni.
Checkpoint 3: Ainsel River
Later, continue through Ainsel River Main, pick up the Miniature Ranni and exhaust its dialogue at a Site of Grace. The route continues toward Nokstella and the Baleful Shadow.
Checkpoint 4: Dark Moon Ring
After the Baleful Shadow, use the key obtained through the quest to open the relevant chest near Rennala in Raya Lucaria's Grand Library and obtain the Dark Moon Ring.
Checkpoint 5: Lake of Rot to Moonlight Altar
Continue through the Lake of Rot and Grand Cloister, defeat Astel, then reach Moonlight Altar and the Cathedral of Manus Celes to finish the main route.
Use a progress note
Because the quest spans many regions, record the last completed checkpoint instead of trying to remember every dialogue line.
Reference: GameSpot Ranni quest guide.