First you must speak with the Dreamer: Stingtail at Fort Joy. He is on the beach area south of Fort Joy. He will only appear after you begin the quest: An Imprisoned Elf and have spoken with Griff in Fort Joy. Sebille will ask to speak to him when you do, and if you say yes she will kill him. He is a Pyrokinetic Skills Book vendor, so be sure you have purchased whichever Skill Books you want from him before this occurs. If she doesn't kill him you cannot advance her quest line. Also, if you tell Griff Stingtail was the thief during the quest: An Imprisoned Elf, then you will not be able to advance this quest. So be sure to either let Sebille kill him first, or don't tell Griff the thief's identity.
Next have her speak with Griff in Fort Joy and she will learn that she needs to speak with Zaleskar in the Hollow Marshes. This means you will need to escape Fort Joy before continuing on her quest.
At Driftwood Tavern, A man named Baran will inform you that Roost is hiding in a nearby sawmill. (you need to have Ifan ben-Mezd in your team)
Roost is hiding in the sawmill, to reach there, you need to get past a lot of lone wolves. (if you have Ifan ben-Mezd on your team, you can let him do the talking) From Roost you will learn the location of your former master, then you need to fight Roost.
If you save Saheila from Roost in The Elven Seer, she will reveal the true identity of Sebille, and give Sebille a choice to be the heart of scions.
Sebille's master is at the Nameless Isle. When you first reach there, she will teach you a song that breaks her master's control. Travel to the Temple of Zorl-Stissa, find the Shadow Prince at Lava Field. Sing the song when Shadow Prince wants to control Sebille.
When Shadow Prince turns hostile, a group of shadow assassins will also appear, defeat them to set Sebille free.
After you killed the Shadow Prince, the mother tree wants to speak to Sebille. Go to the heart of the mother tree (using the flower to teleport), it will ask Sebille to grow to be the next mother tree. If you don't agree with this, the elves will turn hostile and attack you, but Sebille will live.
When you leave the mother tree heart room, Sheila will approach you (if she's still alive), she asked you to kill the mother tree, so all the elves will live freely. Get back to the heart to kill the mother tree, the quest is complete but the remaining elves at the temple will turn hostile.
When you reached the Academy, you need to convince Sebille you are the right one to be the divine. (If you finished her personal quest there is no need of persuasion)
Zaleskar requires a persuasion check to be convinced to talk or will turn hostile. Be aware that when you first meet him Zaleskar will probably be several levels higher than your party, so either wait till you level up or get ready beforehand. If you get to kill Zaleskar he will drop a Withered Heart that may be eaten by Sebille to advance her quest..