Plus, it gives you a quick answer to that dreaded question;
"So what's your book about?"
1. Character: pretty straight forward here, it's your protagonist.
2. Situation: This includes setting and external forces, ie: what kind of life does your MC have? Do they live in a futuristic version of New York or a stately English manor in the late 1800's.
3. Objective: What does your MC want?
4. Opponent: CONFLICT! What is preventing your MC from getting what they want.
5. Disaster: Identify the worst thing that can happen to your characters. It can't be rosy on the road to their goal. There should be something hanging over their heads as they navigate their way through the story.
Now that we're all on the same page, let's CREATE THAT PREMISE!
Basically, you're going to bind all those elements together in one sentence, and that my friends, is the premise of your story.
This is very handy for when people ask you what your story is about, you'll have a quick answer that makes you sound like bourbon and evening jackets.
As an example I'm going to use an idea that's been kicking around in my head for a new YA mystery. Let's start with the 5 major elements.
Character: Drusilla Timmons, 19, fake medium
Objective: restore reputation and get rich
Situation: England mid 1800s
Now, let's put it together using this formula:
Situation > Character > Goal > Opponent > Disaster
When her fake séance is raided, con-artist Drusilla Timmons loses credibility and desperately accepts the task of helping a handsome widower find his bride's murderer, but when she visits the reclusive manor the wife's ghost is suddenly real and the bloody finger is pointing at the husband.
Or you can frame it as a question:
In a reclusive English manor, fake medium Drusilla Timmons needs to pull off the séance of her career to help bring a killer to light, but when the ghosts are suddenly real can Drusilla keep her wits and solve the mystery before she becomes the next victim?
I hope this helps! Have fun writing your premise!