A summary of an essay, I assume?
Been a while since I had to do this, but what I would do is break it down into key points that can be spelled out in a single sentence. Start with a goal of one sentence per paragraph, and look especially at the first sentence of each body paragraph (which should be the topic of that paragraph) and the last sentences of the first and last paragraphs (which should hopefully be the thesis statement and/or conclusion).
Make sure you focus on key points and minimize details and examples given. You'll probably need to keep a couple for clarity, but no more than one per point, and keep it in the same sentence if possible ("[point], such as [example].").
You may have to do more dissecting depending on the length of the essay and the length requirement for your summary, but again, your goal is to go down a level with each iteration. Chapters become sections, sections become paragraphs, paragraphs become sentences. Repeat as needed.