I touched on this a few days ago with query but I'm going to go into more detail today. Making a synopsis is a crucial step in the revision process. Do not put it off until you think everything is shiny. Your synopsis isn't just for your query package - it is for you. The act of writing it (and believe me I do know how terribly tough it is for most of us) will shine a huge light on the holes and problems in your manuscript. Don't let the writing of it be something you dread - think of it as an exploration of the manuscript that you want to do as well as possible before you send it off to strangers (cold business strangers I might add).
There are tonnes of blogs that have written great advice about how to write a synopsis - you can locate most of them on The Writers Knowledge Base here. Enter synopsis in the search engine and watch them pop up. I have used a number of different ways to do it including The Hero's Journey as a model, the three act structure and I don't know how many others. All I know for sure is that it is easy when my structure makes good sense and hard as hell when it doesn't - which is exactly why you want to start this exercise early!
Remember that for the synopsis you do not hold back on the basic plot line. In other words you never write something like "read this novel to find out if Sheila meets the Prince or face plants into her lasagna". Nope. You have to tell it and not to flog a dead horse much but that is what will make you see if you've fully figured out the details.
Here is a photo I took yesterday on the bridge on the way to work - nice big chunks of ice go floating underneath as Little and Grand Lake break-up. On the other side towards the bay it is still pretty froze up but spring is a coming!