How does one write a summary?
Writing a summary requires familiarity with the literature to be summarized. Whatever the material to be summarized -- for example, magazine or journal article, novel, speech, essay, etc. -- the student has no real choice but to carefully peruse the material in question. Otherwise, no viable summary can be produced.
A summary, of course, is a brief description of the source material (e.g., the article, book, etc.) that includes the author's thesis (when appropriate), plot (when appropriate), major characters, and conclusions or outcomes. The operative word there is "brief." Summaries should be very brief, often no more than a single page except in the case of "executive summaries" that accompany lengthy documents like conference proceedings, academic or research institution reports, and so on. In those cases, the summaries can be ten pages, as the underlying document might be hundreds of pages. In the case of a novel, the summary should be less than one page.
Attached below is a link to eNotes' own description of how to write a summary, and it constitutes a good place to begin. Retelling a story in a succinct manner can be more difficult than it at first seems, as the tendency is to provide more information than necessary. The challenge, therefore, lies precisely in the ability to describe the various facets of a work of literature in as little space as possible. That means being able to identify the main points or themes of the underlying document and restating them concisely and clearly.