Ciri becomes empress if all the following conditions are met:
- you do not get the bad ending,
- visit Emhyr with Ciri after the battle of Kaer Morhen,
- and complete Reason of State and kill Dijkstra (Nilfgaard wins the war, Temeria becomes a vassal state)
If you take Ciri to Emhyr and he wins the war, she will become Empress.
If you take her to Emhyr but he does not win the war, she will become a Witcher
If you do not take her to Emhyr, she will become a Witcher,the war outcome doesn't matter.
I think there are other ways for her to become a Witcher'even IF you do take her to Emhyr and he does win the war.When he offers compensation don't take it or she'll feel betrayed thus making her decide to become Empress.