OK, since there seems to be yet more interest in plot planning and prewriting and how to do it, you get still more posts on the subject. This one is on alternate ways of doing plot-related planning; next one will be on the kind of outline you do to submit to an editor and how it is different.
The important thing to remember is that the form of any pre-writing plot plan depends entirely on the needs of the particular writer who is doing the planning. It can be anything from a conglomeration of bits of dialog, scene snippets, and occasional notes that, taken together, indicate where and how the story will proceed from one turn to the next, to a highly organized and systematic scene-by-scene summary, to a series of diagrams that look like a programmer’s flow chart. The only criteria is that it helps that particular writer work out whatever aspects of writing he or she needs to have thoroughly clear and worked out prior to beginning to write. If it doesn’t help, it’s totally optional (I almost said “a waste of time,” but some people have a lot of fun doing elaborate pre-writing stuff, and fun is never a waste of time. So, optional.)
Plot-planning can be done in lots of ways, and the process can be very different from the eventual end product. Some writers get partway through a particular process, and by that point they know everything they need to know to go on with, so they quit planning and start writing with their plan looking half-finished. It isn’t half-finished; it’s as done as they need it to be. See above paragraph.
The actual process of planning the plot depends on where you are starting from and what bits are the bits the writer finds particularly interesting, exciting, and fun. If what got you interested in writing this story is the three main characters and their relationship, then the plot is probably going to need to showcase that in some way, or you are going to lose interest. If you have a very clear idea of a scene – first, last, or in the middle, doesn’t matter – then that is what you start with. If you have a plot skeleton or idea, well, that’s what you start with (and yes, people who start here sometimes also need to do planning).
You then need to think for a minute about how you think. If you are visual, your plot planning may work best if you start or finish with some of the more visual methods; if you are kinesthetic, moving Post-It-Notes around the coffee table may work better, or making little models out of modeling clay or Legos. (Yes, I know people who do these things. No, I won’t tell you who.)
The first phase of plot-planning is usually brainstorming. There are lots of ways to do this, from writing down ideas on notecards (one to a card, so you can rearrange them later) or making lists of events that could happen and reasons they might, to “mind maps” where you construct a sort of spiderweb of possibilities on the page, to actual decision trees where you try to follow the different decision paths as far as they will take you, rather like a Choose Your Own Adventure book only without all the text. One of my friends refers to this as “the spaghetti stage,” meaning the point where you are collecting as much spaghetti as possible and cooking it a bit, so that later you can throw it at the wall and see if it sticks.
Most writers start with at least a few things they know for certain. Usually it’s a combination of character stuff and events – for instance, I know that one of the secondary characters is an unwanted third child, but I haven’t yet decided on the character’s age within a very broad range (somewhere between ten and thirty, depending on what skills I need to make available to my heroine). So you look at the things you know for sure, and at where you think the story is going or has been, and at what you absolutely have to have to get from what you absolutely know to the other parts you absolutely know. Then you start coming up with all sorts of ways you might get from A to C, and all the various possible reasons your characters could have for going this or that way, and all the other possible subplots you can think of.
Eventually, most writers want to take the mass of possibilities and alternatives and consequences they have come up with (“F is M’s grandfather/uncle/biological father/best friend of one of the above?”) and start sorting them out into something more coherent. This has three parts: making decisions, looking at consequences, and making up more missing bits to connect the things you have decided and/or the effects of the consequences.
To put it another way: a plot is a series of decisions that the characters (both the heroes and the villains) make that leads them toward the eventual outcome. Planning the plot in advance involves figuring out at least some of those decisions – both what they are and which way the characters decide – and then looking at the consequences of each decision and the direction it leads (which might be the direction the writer wants the story to go, or a totally wrong direction, or a new and more interesting direction than what the writer initially thought of), and finally filling in whatever missing bits are needed to make a continuous chain from start to finish.
For some writers, all this is really intuitive; others have to grind their way through various mechanical systems in order to jog their backbrains into settling on one main plotline. There are tons of different metaphors for how this process works, like the cooking spaghetti one above. Some writers think of it like putting quilt pieces together in a pattern; others, like stringing beads to make a necklace; others, like putting Legos or Tinker Toys or jigsaw puzzles together. Some work best if they have a really strictly organized framework to fill in, while some prefer to be a lot more free-form. Even for the most highly structured, though, things are still extremely fluid at this stage of the game. Plot points and objectives and problems and characters can fly in and out of the story as more interesting ideas or links occur to the writer. This is why you probably don’t want to write this up in indelible ink…
The chain of cause-and-effect decisions and actions can be as sketchy or as detailed as the particular writer wants and needs it to be. Some writers are happy with a half-page list of five or ten key plot points; others need a hundred pages or more of detailed scene-by-scene developments. Again, the point is to do as much of whatever kind of thing you, the writer, need to give you whatever it takes to get you going and keep you going through to the end of the best novel you are currently capable of turning out.
If all this still seems a little vague, it’s because there is no One True Way to do this. Every writer I know has a different, highly customized system of working (and I count “sit down and surprise myself” as a highly customized system). All I am trying to do here is give a broad-brush overview, because that is about all anybody can do. From here out, the answer is: you’re a writer; you’re supposed to be creative; make up your own system.