When you’ve accumulated enough material through brainstorming, the next step is to select what kind of outline you want. Your outline could be a basic sketch that tells you, roughly, what will happen in the beginning, the middle and the end, and to whom it will happen. You could also make an outline that is a detailed scene-by-scene exploration of your story, complete with cast of characters, setting, plot, and theme details. Of course, an outline could also be anything in between these extremes that you might find useful.
The kind of outline you choose will depend on two primary factors. First, it will depend on your skill and comfort with outlining. If you are the kind of writer who finds outlines to be very frustrating to make and even more frustrating to follow, then a detailed outline is probably not going to be useful to you. If, on the other hand, you are the kind of writer who tends to jump into projects without knowing whether or not the story you have can stretch across the length of a novel, then a more detailed outline will allow you to thoroughly test you ideas before you attempt the story itself, which can save you the greater frustration of a novel you can’t complete.
The second factor will be the needs of your story. Generally, I gravitate towards outlines that are closer to the rough-sketch format. When I am first starting a novel, I tend to have a rough idea of the beginning, middle and end, which I sketch out and then write. Later, after I’ve completed a first draft, I go back and do a scene-by-scene outline as part of the crafting stage. It helps me determine what to keep, what to toss, what to prune, and what to grow. (Keep in mind that I have yet to complete a publishable novel, so my personal experience is not necessarily something you should follow.)
Complex stories with multiple storylines, two or more primary characters, and a need for plot twists tend to require a bit more detail. I’m working on a novel that involves three primary protagonists and three primary antagonists, all of whom affect each other in complex ways. The novel itself is driven by the needs and desires of the three primary protagonists, but the needs and desires also serve as launch-points to get the characters and the readers involved in much bigger stakes. One of the key decisions I have to make is how much knowledge to give the characters and when. This decision, which must be made at many different points, will dramatically impact what the characters do, and how the plot develops. These decisions are both little-picture decisions that affect the outcome of individual scenes where the knowledge may or may not be discovered and big-picture decisions that affect how the protagonists relate to each other and their respective antagonists. Testing out these decisions in a detailed outline will allow me to—however painful it may seem for the writing part of it—choose the appropriate places to withhold information and then, at last, to wrench the characters away from their false targets by providing them with the missing pieces. Without these plot twists, the story is very different and, in a way, far less believable. I have tried to write the story from a bare-bones sketch, but I tend to reveal too much to my characters too soon. So, through outlining, I can see the bigger picture from the beginning—and I can know how to hint at the upcoming twists without revealing them.
There is no right outline length or outline style. Outlining is a tool. It’s most effective purpose is to provide the writer with the means to navigate a novel-length story (though it’s also useful for shorter works and, of course, for nonfiction). It can also be a marketing tool used by a novelist to pitch a completed novel to agents or publishers. At this point, however, we’re talking specifically about an outline written for the writer’s use.
As a writer you need a tool that will serve your purposes. Sometimes you don’t need an outline at all, and your story is one you will discover as you write it. Other times a bare-bones sketch will help you see your way through. But, sometimes, you need that detailed map to navigate your way through the plethora of story elements in order to create the grand vision that might otherwise remain trapped in your mind. A writer’s comfort and skill with outlining is a factor, but the driving force in the outlining decision should be based on what tool the writer needs to tell the story.