Jot down the document's main points or arguments. Not all documents have arguments but almost all documents have a main point or overarching theme that can be summarized in a sentence or two.
Write down the key point of each paragraph. Most paragraphs should reinforce or expand upon the argument or overarching point you found above, providing more specific information but still relating to the main point.
Write down any necessary supporting details if you have room. The more room you have in your summary, the more detail you can provide.
Rewrite the document, using only the information from Steps 1 through 3. You should use your own words to make your new document flow more effectively and connect the ideas. What's more, writing it yourself will make it more memorable to you.