Ideas come. They can start out big. They can start out small. They can start with a character, a plot point, a setting, a theme, or just about anything else that strikes your fancy. But, wherever they start, this initial idea isn’t enough to make an outline let alone the actual novel.
The first stage of outlining consists of brainstorming, which you should do whether or not you outline your novels. Brainstorming can involve actively thinking about what the story might be, or letting the pieces of story percolate and grow in the back of your mind, or a combination of the two.
The purpose of this stage is to produce information. You need to learn more about all aspects of your story. You also need a means of capturing this information. Some writers prefer a notebook or note cards. Others prefer to use software. I like Microsoft OneNote, because it came with my software package and it’s easy to use. (I have not shopped around.) Whatever method of collecting information you use, you need to keep some sort of order. This should make sense to you, but doesn’t need to make sense to anyone else. (If you’re the least bit paranoid, you might feel better if it doesn’t make sense to anyone else, but that’s usually not necessary.)
Maintaining order is not an outline. When you do your initial brainstorming, you won’t have enough information to sort it into an outline and trying to do so may stifle your creativity. Let the pattern of the story emerge more naturally. Then, when you have generated and collected enough information, you’ll be ready to outline.
On the other hand, a character sheet for each primary character (protagonists and antagonists) can be very useful. Setting and plot sheets are also good. You don’t necessarily have to fit them together, but do keep track of what comes to you.
Unless you’re hyper-diligent, some things will slip through the cracks. That’s okay. Either it will resurface or it won’t. Work with what you manage to salvage and be prepared to develop all your ideas further.
There’s no specific time frame for this stage. It can take days or years. I do, however, recommend you keep active on other projects while brainstorming. In my experience, it helps stir things loose from the percolator and get it down in your notes.