How I'm wrtiting fanfic?
I sit down to write. I have the themes, the concept, from which usually flows the structure and the style, I have some dialogue, a couple key scenes, and that's about it. I go back to my first paragraphs and start from there. I write the story from start to finish, usually in chronological order, with a few breaks [that never include work, ever, which doesn't endear fanfic to my colleagues].
The story takes over my life, and there's nothing to be done until it's written. There are a few exceptions. Well, it was difficult in that instance -- I tried to be as honest as I could, as open as I could, not hold anything back, or gloss over the mess that was John -- but the difficulty wasn't technical.
As soon as I'm done, I print the whole thing, and make edits on paper. The edits are mostly deletions (because I tend to ramble), and I go through each verb and adjective to make sure that each word is as close as it can be to what I'm trying to express. My thesaurus is my best friend, although I don't need it much anymore, which comes with reading a lot, I suppose. I also dislike using the same adjective twice in close proximity if I can help it. A lot of it is about rhythm, the number of syllables, the equilibrium of a single sentence, which translates into the equilibrium of a paragraph, and eventually the whole story. The words sound out loud in my mind. Sometimes, they even rhyme. It's important that the mechanics serve the content [sharp, broken sentences for action, run-on babble for a hallucination, etc...].
I send off to the betas, get it back and make the corrections [added to a few more corrections I've made in the meantime]. The only beta suggestions I probably won't take into account are points of characterization. That's not something I feel the need to have a second opinion on. I need beta for grammar, the typos, punctuation [waaay too many commas], a word here and there, and sentences that are much too long at times.
The final draft is never radically different from the first one. I've never had more than three drafts, including the final one. All in all, it's pretty straightforward. Not very intellectualized. The plot just comes. The structure appears out of thin air. Then it sort of writes itself.
So much for interesting insight. It's not unusual for the whole process to occur inside a week. Incidentally, it works exactly the same way when I write original fiction, although it's spread across a longer period of time.