An expository paragraph should begin with a relevant and clear topic sentence. Much like the thesis statement of an essay, the topic sentence encapsulates the main idea or argument of the paragraph, and does so concisely. In a paragraph about baseball, for instance, a good topic sentence would be, "Helmets are necessary for preventing injury in baseball." Avoid unclear or overly general topic sentences, such as, "Helmets are good in baseball" or "Helmets help you when playing baseball."
The topic sentence should be followed by a directional sentence, the purpose of which is to name the evidence you will use to back up your claim. Structure your directional sentence by naming evidence in the order in which it will appear below. In the baseball example, a good directional sentence would read, "They protect batters, base-runners and coaches." Again, be specific and avoid general language that obscures your point, such as, "They help protect many different people on the field."
Now is the time to expand on the evidence listed in your directional sentence. Write one, or possible two, sentences for each of your pieces of evidence, using succinct language that gets right to the point. In the baseball example, the evidence could read as follows: "Batters are exposed to fast-moving pitches that can hit them in the head. Base-runners sometimes suffer collisions with fielders, which can cause a concussion. Coaches stand on the sidelines, but are often in the path of sharply hit foul balls."
Wrap up your expository paragraph by summarizing and moving past your evidence. This can be in the form of a call to action, a broadening of the topic or a lead-in to the next paragraph in the essay. An example of a good conclusion is, "Helmets should be worn by all baseball players, regardless of age or skill level. By insisting that helmets be worn, you can help prevent injury and maybe save a life."