This is a very good question, as it is a skill you will undoubtedly use many times over the course of your career as a student. I assume you are writing the summary as part of an assignment rather than just for your personal study use. If it is for your own use, the simplest and most useful format is an outline, with each of the sections as a main body point.
For a written summary, I have attached a link to an excellent eNotes site below, but here are the highlights:
- Skim for highlighted, bolded headlines and section titles
- Read the material once, just to get a feel for the style, tone, and content
- Reread the material and capture the main idea of each section in a concise sentence
- Write an effective thesis (purpose) statement
- Write, keeping in mind the following guidelines:
- "Write in the present tense.
- Make sure to include the author and title of the work.
- Be concise: a summary should not be equal in length to the original text.
- If you must use the words of the author, cite them.
- Don't put your own opinions, ideas, or interpretations into the summary. The purpose of writing a summary is to accurately represent what the author wanted to say, not to provide a critique."
The last step in any writing process is to check for accuracy and make revisions as needed.