I just finished writing a synopsis for Tris (my agent). I thought to myself, This will be a cinch! Just find a summary somewhere and send it to him. Then I tried writing a whole summary of Cooper's Moon, and wow! It took me a full week!
Nathan Bransford, author and former agent, wrote a compelling piece on writing a synopsis. You can read it on his web site. The essence of what he said was that it's a bloody hard job.
It's harder than writing the book, in my mind, because the synopsis has to say in a few pages what it took you 384 pages to say! As President Lincoln was reported to have said when asked why his address at Gettysburg was so short: I would have made it shorter if I had the time (or something like that).
All I know is what I did: outline the whole book--about 20 pages of headings--then reduce those pages to about 10. The synopsis was still too long. So I cut and slashed and gnashed my teeth until I had it down to about 4 1/2 pages and sent it on to Tris. He said he liked it after cleaning up several items--he is such a good editor. I was relieved and slept for the next three days (slight exaggeration).
So my recipe for writing a synopsis: outline each chapter, reduce the outline, then reduce it once again, and finally put the whole thing in an exciting, moving narrative. There you have it. The process could be titled, How to spend a week without eating or sleeping. I hope you never have to write one. But if you do!................