Neurotic, Hates the Outdoors, and/or No Sense of Humor. Hopefully you have a moodlet manager or motive mobile by then too, just to make things even easier. Getting stressed is definitely the hardest part of writing.
Other than that, it's not that hard, it's just a lot of tedium. For the supermax, anyways. If charisma is a free skill, I'd pop them into Journalism instead as a career. Pam had to do that at the last minute with Forest. The self-employed writing career is certainly possible, since saltpastillen and Rica both did it, but it requires starting them as self-employed from teenagerhood and basically doing nothing else but write.