You have several choices, and which one you use depends on the length of the document to be written, the subject matter, and your personal preferences. All sections here are examples of the technique they describe.
The outline of this answer would look like this:
- Introduction - mention options
- Outlining - explain, give example
- Infodump - vomit it all onto the page, organize, edit
- [x]-paragraph essay
- Combinations, summary
You would then go back and use this as a guide to write the sections. (Keen eyes will notice I didn't stick to this: I decided after writing all this out that section 3 worked better after section 4, since it relies on having more experience with writing. But it was a good place to start.)
The [x]-paragraph essay
We learned this in high school (possibly earlier), and it's a great way to get used to writing in an organized way. It forces you to think about what you want to say, but it can be a bit of a straitjacket. The first one we were taught was the three-paragraph essay.
The first paragraph is where you write the basic idea, very much a generalization of the ideas you want to get across. You're also framing the rest of the essay, creating a lens through which the reader will see all subsequent text. Note that you're reading this with foreknowledge that it's a technique taught to children, since I mentioned that in the above paragraph. Decades after leaving school, I still find the technique useful for smaller pieces. Even if I usually don't stick to a set number of paragraphs.
The three-paragraph essay scales well to the five-paragraph essay, and beyond. I wrote this section as a three-paragraph essay, but that middle paragraph really wants to be two paragraphs - but it's a good place to start. Thinking about writing in this way can discipline you well.
Write like a madman, but sort it out later
While writing an outline can help for some people, and having a paragraph structure in mind works for some stuff, these don't work for everybody or at all times.
When the outline approach isn't working--it rarely does for me with anything less than a few thousand words--an infodump may be what you need.
Type out everything you want to say. Don't worry about structure yet, just write out all the points you want to make. Then, read it over and decide what the main points are you want to say. Write a introduction and/or conclusion, as appropriate, then organize the rest of it into the "middle" of the piece. Summarize, proofread, and you're done.
This may not seem like much of a writing technique. What you're doing here is giving yourself permission to freewrite on a topic, with the knowledge that you'll organize it all later. Great for short projects or more creative work.
You'll probably find yourself picking and choosing from these techniques, and possibly others as well. My favorite technique is the infodump combined with some quick outlining, although I pay careful attention to paragraph length and structure along the way.
In the end, try different tools, use the ones that work, and let them influence your thinking. Eventually, you'll do much of this without thinking about it. Using disciplines like these trains your brain.