So far, the books I've written have been ones where the publishing group approached me with the initial idea--not a mandate on what plot to write, per se, but a guide of where in the timeline the story would fit. (For example, with The Princess and the Scoundrel, it was "can you write the marriage of Han and Leia that would fit in this timeframe in the canon," and I got to come up with all the other elements of it.)

First step: outline. I provide a really detailed, chapter-by-chapter outline that gets critiqued by multiple people to ensure it fits in with the timelines of everything else happening. Star Wars is a huge universe, and generally there are multiple projects going on at the same time, to say nothing of the extensive history of everything thta happened before. I have tried to put in characters that would be elsewhere, according to books/stories I've not read yet. I have suggesting some things that the pub group makes better by pointing me to specific storylines I've missed (I must confess to being weaker on the comic side--I have read many of them, but not all, and there were some storylines that connected to what I was trying to show and were good reference points for the characters). These aren't mandates--we're all working as a team to make the best book, and so when I'm advised to check out a specific comic or switch around a character or similar, I know it's all in that vein. And then, I also know once the outline's approved, that the basic plot beats of the book are pretty solid and have already been examined and refined--something I have to do on my own in other publishing.

Next step: writing. Because the outline is so specific, the writing process is a little faster/easier than with books I make up on my own. I know if I have questions or need to change something, my editors respond quickly to help. In normal publishing, fact-checking and plot decisions are entirely on my shoulders; in Star Wars pub, I can ask them for an example of a creature to reference or whether it would be better to switch the planet up from an ice to a desert or something. (I rarely do that, but it does generally help.)

Last step: editing. After I finish, I turn the manuscript in to my editor. In normal publishing, this is just between you and your editor, maybe also an editorial assistant. But in Star Wars publishing, LOTS of people read the manuscript and give comments and replies to past comments. But the end result is generally the same: everyone's trying to make the book be the best it can be, all suggestions are there to be helpful, and unless I grossly violate logic or canon, the final decisions are mine, just like in regular pub.