An outline is the organizational tool we have for any topic. With this tool, you can make your talk, book, or topic flow from one idea to another. Just like the paragraph, the outline is the basis for reasonable dissemination of ideas from one to another.
To make an outline, start with the topic. This is the idea you are trying to convey.
I. Then you start to cover the first main idea that you wish to discuss.
A. This is the first point's idea
B. This is the second point of your first main idea.
C. This is the third point of the first main idea.
II. Now you have a second point of the topic to cover.
A. The second point's first idea
1. Something very complicated about the second point's idea
2. The second complication of the second point's idea
B. The second point's second idea
III. The third point to your discussion
IV. The conclusion to the topic.
Notice that the Roman numerals, letters, and numbers will be a diagram of the topic in depth. This is the "roadmap" of your topic and those who read or listen to your ideas have some clue to the direction of your ideas.
This is the way that you "map" how you are going to write your speech or paper. The outline can help you keep from adding things that don't need to be there, and can show you how you get off track when you add in things that might be related. It's a tool like any other in writing.
One of the "rules" to outlining is that (1) the big points are Roman numerals, (2) the ideas in the big points are alphabets and (3) the sub points are numbers. If you need more organization than this, add lower case letter headings to the numbers, then numbers again to break it down even further. For most people the outline above is sufficient to do the job.