Let’s begin with what a paragraph is. In an academic setting, a paragraph is a composition of at least six sentences and at most two-hundred words that says one thing and supports, elaborates or defends it. That’s a rule of thumb and it doesn’t mind being broken now and then. (The rule not the thumb.) And it may be adjusted for different disciplines. There are areas of scholarship where paragraphs are generally shorter than that, and even some where they are longer, but for most purposes, in most of the humanities and social sciences, approaching the paragraph on these terms will work. Certainly, if you are able to compose paragraphs like this, you are able to do a great many other things, including writing somewhat longer or shorter ones. The important thing is that each paragraph has a single, well-defined point, a deliberate rhetorical posture, and a finite volume. If you need an example, this is the last sentence of the first paragraph in this post.
The first order of business is to decide what you want your paragraph to say. This is best done by working out a “key sentence” that neatly summarizes what the paragraph is about. It doesn’t have to be the first sentence of the paragraph, though it often is; it just has to somehow stand out as the sentence that makes your point. It will usually be a simple, declarative sentence that states your meaning plainly and directly. Obviously, this sentence can be revised as you work on the paragraph, but you want to have a version of it in front of you when you begin. It should say something you know well enough to write a whole paragraph about. It should never express everything you know about the subject; it shouldn’t exhaust your knowledge. It should have the dignity of an iceberg, as Hemingway puts it. When you look at it, you should be aware of all the knowledge that you have waiting under the surface.
I recommend you decide on the key sentence the day before you write the paragraph. At the end of the day, when you have decided that you’re not going to learn anything new, that you’re not going to get any smarter, take five minutes to call to mind something you know to be true. Write it down in the form of a good, strong sentence and take a quick moment to gauge its weight. How does it feel to you? Is it a point you’d struggle to support, elaborate or defend in the company of one of your peers? Would you have a hard time providing evidence, explaining your meaning, or dealing with objections? Or is it comfortably part of what you know? If you feel confident about it then resolve to write a paragraph about it in the morning, to exercise that confidence in writing. Pick a specific time and place to do it and then — and this is very important — put it out of your mind until that moment arrives.
The next day, sit down at the appointed time, in the appointed place, and type out the key sentence you wrote last night. Now, think of your reader (a peer in your discipline or a student in your class). What difficulty does the key sentence pose for your reader? Will your reader find it hard to believe, hard to understand, or hard to agree with? I recommend you keep it simple; confine yourself to a single difficulty and spend the entire paragraph supporting your point (if your reader finds it hard to believe), elaborating it (if your reader finds it hard to understand) or defending it (if your reader finds it hard to agree). It is possible to shift your posture in mid-paragraph, but this is, let’s say, an “advanced” skill. Get the basics down and then move on to harder, more complex, problems, or even “the fourth difficulty”: boredom. In this post, I have limited myself entirely to explaining what I mean.* In others, you will find I engage with readers who disagree with me or find what I say hard to believe or fail to see why I’m so excited about all of this.
This is both an exercise and a practice. You can get through much of the writing you need to do for your school or research career by writing paragraphs as deliberately as I’ve here suggested. But working in this way will help especially when you’re trying to improve your writing. It will make your difficulty as a writer explicit and it will bring the resources you can bring to bear on the problem into view. Give yourself 18 or 27 minutes to write each paragraph. Notice that this gives you 18 or 27 times longer to write the paragraph than your reader is likely to have to read it. (It takes about one minute to read 200 words. It should have taken you about five minutes to read this post.) Spend the first two or three minutes on your key sentence, then spend about ten minutes composing additional sentences. Then spend five or ten minutes making the sentences as clear and concise as possible. Then read the paragraph out loud and spend the remainder of your time fixing whatever that showed you needed fixing. Then stop. Every time you compose a paragraph like this you will become just a little better at writing.
*Update (25.11.21): Rereading this post, I wonder if that is entirely right. The opening paragraph for example: does it really only elaborate my meaning? Is there not something, if you will, defensive about it?