If you're looking for the short synopsis that goes in the query letter, focus on the basics of what the story is about:
The overall problem that needs to be resolved by the end; what bad thing will happen if the main character isn't successful in solving the problem; and a major conflict that gets in the way. You really want to get the core story down in one or two sentences and then build the query synopsis out from it. The query should end on a hook that makes the query reader want to ask to see a partial. Limit it to one or two characters; otherwise, you spend all your query real estate introducing characters and not talking about the story.
If you're trying to do a five page synopsis, this is something that worked for me. I went through each chapter and wrote down just a sentence or two about the major story event that occurs in that chapter. Then I began merging them together. Some came out because, while they were important in the story, they weren't necessary to get the overall story in the synopsis. The beginning, middle, and end should be included. Limit it to no more than four characters. Again, you don't want too many characters cluttering up the synopsis and distract the reader from the story.
And one thing you can do to really help yourself with learning how to do a synopsis is to do crits of them on the SYW board.