I tend to assume that we all are taught outlining at a relatively early age, so it seems, perhaps, a little obvious to talk about how much it can help in the writing process.
But then I work with people who have trouble putting it to use, and I reflect that, I too, though taught to outline, had trouble with the practice for a long time.
Nowadays I like outlining, but only the most basic of outlines. I try to avoid long, highly detailed outlines of a whole work. I find it too easy to get lost in detail. Instead I just make simple out lines that help me see the overall arc of the project I'm working on. Often I keep my outlines to one or two levels. Usually if I'm making an outline with more levels, I find that it's hard to finish--I get lost.
By keeping the outlines short and sparse, it makes outlining a simple task--one that can be accomplished in a short time.
I can add detail later. Often I add detail when I'm working on one of the sections of the higher-level outline. I make a simple, two-level outline, and use that to guide my efforts. But then, when I start to write a single part of that simple outline, I make another simple outline of the one part that I'm working on--again it can be a simple two-level outline, but it has more detail because it's more focused on a specific part of the paper. Again, I want the process of outlining to be simple and quick, so I can get on to writing.
Outlining helps me see what the overall picture is, which helps me keep the writing relating back to the main points and not drifting off into too much detail.
I also like to estimate pages for each piece of the outline. For whatever reason, I rarely meet anyone who does this. It's amazing to me how much adding pages to the outline helps me see the project as more manageable. Suppose you have to write a paper that you estimate will be 100 pages. That sounds like a lot, to me, at least. But if I have a simple outline that breaks the paper into five pieces, I can see that each piece will be around twenty pages--maybe some longer, others shorter. But twenty pages doesn't seem intimidating--or not as intimidating as 100. If I then break the twenty page paper up in a simple one-level outline with four parts, I can see that I have to write four related five-page papers. And five-page papers aren't intimidating at all. If you break it up further and further, suddenly you see that you have to write a page on this subject and two pages on that--and none of those little pieces are intimidating.
And yet, if you're working from outlines that help you see the overview, and help you see how each piece relates to the main project, those little pieces won't be details drifting off into nowhere, they'll be anchored in the basic ideas that drive the whole project.
At least that's how I like to use outlines to help me make a paper feel more manageable while still being able to see the overview of the project.
Of course, that makes it sound simple, but it rarely is. Writing on a complicated subject is complicated. As you try to make outlines, each one reveals strengths and weaknesses. Different structures create different opportunities and different obstacles. Working through that is part of the learning process that is writing.