The point of the synopsis is to tell the whole story of an ms in a way that interests the reader (a literary agent or publisher you want to wow) in one page. A good synopsis can lead to a request to read a partial or full of your ms. A synopsis is a combination of book report (remember those?) and a book review, but a bit different.
The format is universal with a few personal variances per agency or publisher. The sensible approach is to go with the basic format, then make the changes necessary to comply with the specific preferences of your target.
Start with setting your word processing software like this:
*One inch margins all around (top, bottom, sides). One inch is 2.54 cm.
*Single space lines.
*Font is usually Times New Roman or Courier, 10-12pt. I like Tahoma 11pt.
*Center the title, which is Synopsis: title. (If you manage to get the job done in less than a page, then put synopsis and the title on two lines - followed by a blank line.)
*Word count. This is generated by the software, but you'll have to type it in where it goes. There is a mathematical formula for estimating word count. I generally use 300 words per page and do a ballpark of how many chapters end short of a half page. I bet there is an A4 formula out there somewhere, but I've never needed to use it.
What to write:
*Regardless of the POV of the ms, a synopsis is always written in third person.
*Focus on the main character (MC), not all the minor characters. Indicate the MC's name, but honestly, unless they are referred to by their first and middle name by everyone in the ms, then no one cares what their middle name is. For example, I have two friends named Mary Ann. One goes by Mary Ann and the other by Mary. On the second one, no one would care about the middle name if she was the MC.
*Tell the setting, genre, time (contemporary, historical or future -- by indicating the date)
*Tell the whole story. Write the beginning, middle, end of the story. (YES! Tell the end of the story.)
*Tell only the major twists.
*Use active language, rather than passive.
*Don't get poetic, cute, or clever.
*Do not talk to the reader, ie, using second person, YOU. "You will love this book" NO. "You will be scared spitless." NO!
*Don't pitch. None of this: Everyone will love this book. That's a bad pitch anyway, but pitches don't belong in a synopsis.
If I help someone with their synopsis, I want them to tell me nothing about the story before I read the synopsis. The synopsis has to stand on its own. It has to tell the story honestly, but engagingly. Cut out the non-essential rambling. Some people accept a two page synopsis, but if you can do it in one page, then you have done it correctly. The standard is one page. The grammar and punctuation should be perfect.
Give the synopsis to a few reader friends. Listen closely to the questions they ask after reading it. It is likely the agent or publisher will have those questions too. Don't dismiss the questions. Fix the gaps. There is only one shot per target at getting this right.