Concentrate on your first and last paragraphs. The first should contain all of the relevant information from your first chapter–you’re setting up the world, identifying the characters, and setting off the inciting event.
The last should be a condensation of the end of your book–resolving plot points, finishing character arcs, and mentioning any issues that weren’t resolved.
The middle should be every other major event.
Depending on who’s reading your synopsis, you only have 500 to 1000 words to work with. If you can’t fit the middle of your book into your middle paragraphs, then you gave too much detail somewhere.
Use 3rd person (usually), and keep your writing active. Leave out anything that isn’t very important to the section you’re writing about, but you will want to include feelings and concerns, especially when they’re motivations for later actions.