When I first started writing, I really didn't plan. All I would do is get the basic idea and go for it, having no idea about characters, setting, plot, or anything of that nature. But now things have changed, and I plan with more attention.
I start with the basic idea: characters and the situation. I give the characters their goal, or where I want to story to end up. Then I write a very loose outline of the chapters, where I figure out what happens in each chapter, so I know if I have filler or if every event adds to the plot/develops the characters. After that, I give a simple order of events to happen in each chapter before I write it (so only one chapter is planned at a time), but any further details are completely unknown, and I'm still never sure if there will be a change to the plan until it's written.
Right now, I can manage a chapter each week, which includes planning the chapter and writing it. (Editing comes later.) But the chapters for this fic are on the short side (eleven pages). Other fics have much longer chapters, which I know will take longer.
I should mention that I have one story that I keep completely unplanned. When I write that one, I don't know what's going to happen. All I know are the characters and where they're going to end up.
Like Astinus, I thought of the characters and their goals along with the situation. I then thought of a beginning and end while only thinking a few scenes in the middle. I might be able to plan what I want happened in each chapter, but half the time I end up needing to shift a few scenes I want into the next chapter.
Back when I did Nothing, Everything I actually had the draft of the whole story done in a month and then it would take me anywhere from a couple weeks to a month to get each chapter edited (part of it is I would have a beta or two looking over also). For the story I'm doing now the chapters I'm able to finish within a week and I'm already done with the rough draft of it, but the editing is taking much longer as my betas kept suggesting changes and I kept wanting them to look at the chapter one more time, haha.
I write the entire story in rather detailed outlines for each chapter. I have teaked a bit around as I wrote, but I never really move away from my specific 'plan'. I write each outline by chapter, scene, and character.
I also sometimes write everything by hand first before typing it out.
Also I've found that the program ywriter is particularly usefull.
Yes I do tend to plan ahead. For example, in NaNo2011 I went in with only a brief synopsis and a general feel of the events just in my head; nothing was written down bar the one paragraph synopsis, which itself was very vague. I found that I didn't really have any sense of direction and just wrote whatever I wanted, and although one could argue that that tends to happen regardless in NaNo anyway due to the time pressures, I have found in other writing endeavours that a lack of a plan really does stifle my progress. I feel it's very important to have some sort of idea - written down, preferably - of both the start and end, but most importantly the events in the middle, because if not the story can fall apart.
"Umbreon evolved as a result of exposure to the moon's waves. It hides silently in darkness and waits for its foes to make a move. The rings on its body glow when it leaps to attack."
I tend to have an outline of the premise and plot of the story I'm writing, as well as more detailed plans (or sometimes just vague, slightly rambly, notes) for particular events I want to happen within the story. And then, as I write, I find a way to link all of this together. I also sometimes write out separate plans for my characters, just so I have something to refer back to, and keep my characterisation continuous and even. I find this especially helpful when a piece is going to have a lot of important characters.
Before you ever start writing a story, you have a plan. How long does it take for you to fully flesh out a story? How comprehensively do you plan?
Me? Plan? Don't make me laugh.
My first (and practically only) fan fiction was just me pouring out ideas onto paper. Things just happened and I kept going. I like to surprise myself. Whenever I start planning things the story gets stagnant, because I know what's going to happen.