It sounds so easy when they say it.
If you think summarizing a 300-page novel while maintaining voice, keeping the writing engaging, and putting in enough information (but not too much!) to see how all the pieces of the plot go together sounds like about as much fun as a root canal, you're not alone.
Oh, and did I mention agents and editors often want the synopsis to be no more than 1-2 pages?
If you start looking like the dude in the picture at the very thought, read on for a few handy tips and sites that might help make the synopsis-writing process more bearable.
First of all, it's very smart to have two versions of your synopsis, a long one (4-7 pages is safe) and a short one (1-2 pages). In my experience, most agents are busy these days and want the short one, but I have had a few ask for the one with the most detail.
Most advice will tell you to write the long synopsis first and then condense that to get your short one, but I found exactly the opposite worked for me. Once I had the short one together, it was a snap to add details, characters, and plot points to flesh out the long one.
Here is the absolute best site I've ever found for writing a short synopsis: http://www.publishingcrawl.com/2012/04/17/how-to-write-a-1-page-synopsis/. Susan Dennard gives you a worksheet with specific questions and uses Star Wars as an example as she walks you through each step. I cannot recommend this method highly enough. I've now sent this link to a half-dozen writers, and each person has commented on the simplicity and effectiveness of this method.
A few other sites you might find useful:
Here's a good resource for trimming a synopsis that's too long (a common problem!):
Good luck with your synopsis, and feel free to ask questions in the comments!