Sometimes I'm not certain why writing a synopsis is such a difficult task. Are you excited about your manuscript? If you are, write down what you would tell a friend about your story in hopes of getting them interested in reading it. Take those notes and make bullet points. Organize them in a way that makes sense. Start there.
Most synopses I see are brain dumps. They are completely disorganized, mentioning things that have little impact on the story, and do not delineate an actual plot. What is the conflict? Who is the protagonist? Who is the antagonist? What is the goal? What are the consequences for failure?
Science fiction/fantasy pieces have been the worst, frankly. I've gotten dizzy reading them. What planet is this on? What year? And the words that are made up to represent something are strewn throughout the synopsis with no explanation, i.e. "The Wipcleat escaped from Tootoomootoo." What is a Wipcleat? Just because the writer knows it inside and out, sometimes an explanation can be forgotten. Is the Wipcleat an organism? The leader of a rebellious sect? An emperor? And what is Tootoomootoo? A planet? A prison? A country?
A synopsis is the first thing an agent or publisher reads. It represents your work and hopefully entices the person to read your story. Make sure to put your best foot forward.