Read through your story and as you go through it, jot down basic ideas on a separate sheet of paper. Basic ideas should include who the story is about or is happening to, what they are doing or have done, where and why, and the final outcome of those actions.
Trim the list of basic ideas to only the main points. Any points or ideas in the story that are not absolutely necessary to understanding the story should be removed. If understanding of the story does not hinge upon a point you wrote down, it should not be in your summary.
Write your summary. Keep it as short and simple as possible. A summary should not rewrite the entire story but should rouch only briefly on the main ideas. Avoid adding information on subplots that are not ultimately important to the final outcome of the story.
Have someone who is not familiar with your story read over your written summary. When they have finished, have them explain to you what the story is about. If their answer correlates to your story, you have completed your summary successfully. If they are at all confused or get the wrong idea from your summary, rewrite it accordingly.