Here’s a sample synopsis for my short story Blood Sisters. Obviously writing a synopsis for a short story is very different from writing a synopsis for a novel, but it’ll give you the idea. You might like to read the story first before continuing here.
The original story is 3800 words, and my first attempt at a synopsis was 220 words:
Dubliner Tara McGoldrick sees the same woman every day, and is intrigued by her joie-de-vivre. She would love to make friends with the woman, but doesn’t know how to approach her.
Because of her fascination with the woman she sees a wall start falling, and so is able to push the woman out of harm’s way. But Tara injures her own foot in the process.
Tara and the woman are brought to hospital, and on the way Tara learns that the woman is called Jennnifer - just like Tara’s own daughter!
Once at the hospital the women are separated. Tara discovers that she’s broken her ankle and needs an operation.
Jennifer - who was fine, and was let home that afternoon - comes in after the operation to see her. She asks Tara if they can go to dinner together once Tara’s well enough, that she’d like to get to know her better. Also in conversation she says that she is so grateful that Tara saved her life. Tara explains that she too had her life saved once by the kindness of strangers, by a blood donation after her daughter was born. And in that conversation they learn that it was Jennifer who was donor; that each has saved the other’s life; that they’re destined to be friends and indeed that they’re blood sisters.
That was far too long. So I trimmed it even more and now it’s only 80 words, which is a bit more like it:
Dubliner Tara McGoldrick would love to be friends with a woman she sees each day, but she has no way to approach her. Then she saves that woman’s life, slightly injuring herself in the process. Talking together later in hospital, Tara gets her wish for friendship when the woman invites her to dinner. But even more, in conversation they learn that that woman saved her, Tara’s, life through a blood donation. And so, even more than friends, they’re blood sisters.
Do you see how in each case of this sample synopsis the writing got tighter and the detail became less, whilst still focusing on the important elements of the story? That's what you're aiming for when you're writing a synopsis.