Read the original text and underline or make note of the key points and issues. Don't overdo it -- if you highlight every second word it defeats the purpose, as you need to be able to quickly look back and see what the key ideas were.
Look at the "bigger picture" and identify the key arguments, ideas and themes of the text as a whole. This will be easier now that you've underlined the main ideas within the sentences and paragraphs.
Divide the text into a few sections. This can be done in outline form, by drawing a diagram or any other method you choose. Each section should address the underlying theme in a different way.
Write a couple sentences for each of the sections. Each sentence should summarize its main idea quickly and efficiently; it should stand alone rather than force the reader to look up the text itself.
Write a sentence that summarizes the text's main theme or idea.
Combine your sentences into a paragraph or two. It should flow naturally and give the reader an idea of what the text says, how it says it and what its implications are. Remember that less is more -- you need enough information to identify these things, but you don't need many details, as it is a summary.