A quick TL;DR:
MC met the Prince when they were young, he left such a good impression on her that she wrote fanfiction inspired by him. Some merchant dude found her papers and published them to make quick money, which in turn made the whole country start being fans of the Prince as well (even though he had been seen negatively before thanks to the Crown Prince's family).
To help his position, the Prince's twin sister decided to hire her to write a sequel so more people would continue growing their interest in the Prince.
The Crown Prince's family is set to keep that from happening, we're unsure about the Crown Prince's opinion about it yet (he used to be close to the Prince when they were young).