Writing a summary is really not all that difficult to do. You might be writing a summary of a chapter or an entire literary text, or you might be writing a summary of an entire non-fiction work or an article. No matter what, the steps are basically the same.
You cannot write an effective summary of anything without actively reading it. By active reading, I mean reading with a highlighter, sticky notes, or a pencil and paper to write on. Take notes as you read, looking for the author's main idea and supporting evidence for a non-fiction work and for the setting, characters, and plot in a fictional text. You are likely to have to read the work more than once to do this well. For a very long text, I would organize this perhaps one section or chapter at a time. These are the essence of a summary.
As you write your summary, it is vital that you name the text you are summarizing and the name of the author in the very first paragraph, your introduction, so the reader knows what it is you are summarizing. It is not good, for example, to begin with "In this article, the author says...." The reader does not know what article you are writing about! If you are expected to use APA style, you also will need to provide the year of publication, like this:
"How to Effectively Write a Summary," by John Doe (2011) consists of ten steps.
For MLA style, you do not need a year of publication.
Your introductory paragraph should also include a thesis statement, but this thesis statement needs to be a bit different from the kind you would ordinarily use. In most essay writing, a thesis statement is meant to state our main idea and supporting points on a subject, but for a summary, the thesis statement should state the author's main idea and supporting points. A summary is not meant to share your thoughts on the text, just what the text consists of. This is a significant difference between a summary and a literary analysis. So, for a summary of a non-fiction work, you might have a thesis statement like this:
Smith's article argues that amnesty should be provided for undocumented aliens because it will help our economy and free up our law enforcement resources.
For a literary work, you might have a thesis statement like this:
The Chrysalids is a dytopian novel that takes place in a post-apocalyptic future, where some nuclear event has created mutations that place mutated and non-mutated characters in conflict, the non-mutated having created a theology of "normalcy" that creates a society of outcasts.
This states the basic setting, characters, and conflict of the story, and I can use this thesis statement as my outline to summarize in my body paragraphs.
Now, even though I have summarized the book in just one sentence, that is not quite enough for a summary. You are expected to write more than that! What you must do is distill the entire text, so that you can summarize each of the elements in perhaps three to five sentences. You must be sure not to get bogged down in too many details. For example, it is not so important to describe what each character looks like. And it is not so important to describe every element of the setting. You can name the main characters, for example, in a sentence or two, and explain what role each has in the story. One might be the hero of the story and another might be the villain. There might be one important element in the setting, for example, that it is very cold or very barren, an important element in how the characters behave. But the idea is to just select the most important elements to describe the setting, characters, and plot.
For a summary of The Chrysalids, using the thesis statement I have above, I would have one body paragraph on setting, one body paragraph on characters, and a third body paragraph on the conflict and resolution. Each of these can be explained in five or fewer sentences.
For a non-fiction text, using the sample thesis statement from above, I would have two body paragraphs, again, each five or fewer sentences. One would be about the economic argument and the other would be about law enforcement resources. For each, I would summarize the evidence the author uses to support his main idea.
Finally, a concluding paragraph should remind the reader what the author's main idea is and what support he or she used for that idea or how the literary text carried out the author's ideas.
As you write a summary, you must also be sure to frame it so the reader understands that these are the author's ideas, not yours. This is especially important for non-fiction texts. You cannot make an assertion that will confuse the reader, for example, saying, "Undocumented immigrants should be given amnesty." You have to say something like "Smith argues that undocumented immigrants should be given amnesty. Each time you present an idea or information from the author, it must be clear that it is the author's ideas and information that are being presented.
It really is not difficult to write a summary, and doing so gives you power in your knowledge and understanding of what you have read. It forces you to read carefully, closely, and actively, which is the best way to read.