Here's another piece of advice writing teachers tend to hand out: always do a story outline before you write.
To which I say -- maybe.
Maybe you're the kind of writer for whom this is necessary; maybe this is the kind of story with so many convoluted and intersecting plotlines that you need an outline to keep things clear in your own head.
But maybe you'll find that making a story outline destroys any interest you have in writing the story. That an outline corsets your characters and prevents them from taking on life and leading the story in a direction that you'd never, ever planned, and yet is SO right.
If you do decide to make a plan or story outline, it is crucial to understand that it is just a guide and that it must never be followed slavishly.
I've written novels for which I've never done a story outline (not on paper, anyway -- although I always have a sense in my head of the arc of the story and how the tension must build) and ones where I've done fairly detailed outlines.
When writing fantasy or mystery, I've found a general outline useful because it's a way for me to keep interweaving plots, and the motives behind all my various characters' actions straight. (Yes, if the story is to make sense, every character must have a believable motive for his/her actions.)
I've also found an outline useful as a way to try and capture the feel or atmosphere of the story once I think I have it right. Usually, I will go for a walk (many walks, actually!) to pound out ideas, and to try and move the trajectory of the story forward in my head. Then I jot down notes -- snippets of ideas and snatches of dialogue as they come to me. Once I feel that I have all the pieces, and that they fit, and I have a sense of the atmosphere and the voice of the story, I may write an outline, just for the relief of knowing I have that as a reference in case I forget a small piece of motivation, or plot detail, or some such thing.
But inevitably, I have found that once I start to write the story will go off on a trajectory that I hadn't planned -- but that is right. Well, right enough for that draft, anyway.
Some writing teachers suggest making a chapter by chapter outline. Some writers I know do this.
You have to find what works best for you. I couldn't bear to do a chapter by chapter outline because it would bore me to death to write the story. I like to discover and explore as I write and if I have every event and detail pinned down in the outline, I think I'd find it a slog to actually write the story. I'd just lose interest in it. But that's me.
To outline or not is something each writer must decide for her/himself. It may even vary from story to story.