Henry Farrell, among other things also tells how to structure your essay at all these three levels:
You should structure your essay at three levels.
This is the broad structure of the essay itself. Unless you feel very comfortable that you are an excellent writer, it is usually best to stick to the traditional frame of an introductory section, a main body, and a conclusion. The introduction tells the reader what you are going to say. The main body tells the reader what you are saying. The conclusions tell the reader what she has just read (perhaps adding some thoughts as to its broader implications if you are feeling adventurous).
This not only helps the reader understand your argument, but disciplines your thought and prose. It forces you to begin your essay with a clear, concise account of your major claims. When you write the main section of the essay (or re-write it, as needs be) the introduction will provide you with a roadmap of what you need to do. Your conclusions, in contrast, should draw the threads together, showing how the facts and arguments you have laid out in the main body actually speak to the broad themes discussed in the introduction, and drawing the threads of your narrative together into a proper whole. Of course, for this to work it is necessary that the main body of your essay actually speak to the arguments laid out in your introduction, that your conclusions relate to the main body, and so on.
This is perhaps the most commonly neglected element of structured writing. It concerns the paragraphs into which your prose is organized. Each paragraph should focus on one main point. The point of each paragraph should build on that in the previous paragraph, and create the foundations of the next. Each paragraph should be a necessary part of the overall structure of your essay.
I find that a useful mental exercise is to boil down the arguments of each paragraph, one after the other, into single sentences. Then, put all these sentences together into a consecutive narrative, looking to see whether each sentence can be made to flow naturally from the sentence previous to it, and into the sentence following. This will highlight any major structural problems. If you are not able to boil down each of the paragraphs into a single sentence summary (however simplistic), then the offending paragraphs most likely need to be rewritten more clearly. If there are gaps or non-sequiturs when you put the one sentence summaries together, then the meso-structure of your essay needs to be re-organized, by cutting and pasting paragraphs, or by introducing new paragraphs to fill the gaps, or deleting old paragraphs that detract from the flow of your argument.
What is true of the paragraph is also true of the sentence. Each individual sentence should flow in a logical and obvious way from the sentence before, and into the sentence after. Consider the following paragraph, taken from a term paper on global warming which is available for free online.
Weather these days has become very unpredictable. The increase in the world’s temperatures, believed to be caused in part by the greenhouse effect which is known as global warming has and will have a serious effect on the future. Global warming creates massive concerns for the entire earth. If the heat continues to increase several species may struggle to survive. There are numerous political, environmental, economic, and social issues when it comes to global warming. Global warming is an inevitable issue and by no stretch of the imagination can be slowed down easily. There is an inconceivable amount of causes that connect to global warming.
This is quite wretched writing. The first sentence is a vague generality that does not mean very much. The second sentence does not flow in any obvious way from the first. What does the greenhouse effect have to do with unpredictable weather? No explanation is provided for the reader. The third sentence merely repeats the argument of the second, with greater rhetorical alarm. The fourth does a little better, but loses force because it is so badly written (the claim that ‘several species’ may struggle to survive suggests that only five or six species are in danger, which sits awkwardly with the previous sentence’s suggestion that global warming causes “massive concerns” for the entire earth). The fifth sentence seems to build a new set of claims, and should be at the beginning of a new paragraph. However, it never goes anywhere. Instead, the sixth sentence warns that global warming is “an inevitable issue” (whatever that means), while the seventh sentence wrings its hands in despair over yet another new claim – that there is an “inconceivable amount” (sic) of causes “that connect” to global warming. These sentences are not only bad in themselves – they are not connected in any logical or orderly way. The result is that they do not add up to a coherent argument.