fiction - How to derive a first sentence from a story?

There is much advice out there on "how to write a killer opening line". Usually these blog posts or how-to-write book chapters list examples of first sentences from recognized masterworks or group them into categories such as:

1. A statement of eternal principle


2. A statement of simple fact


3. A statement of paired facts

and so on.

Using that advice, I can come up with a hundred intriguing opening sentences, none of which fit my book. Because what none of the advice out there tells me is:

How to write a killer opening line for my book. Not just some random opening for a non-existent book, but one that opens my story.

To find a first sentence for a story I have plotted, I cannot simply use the advice to write "a statement of eternal principle". A sentence such as "The sun rises in the east" does not fit many books, although it is a statement of an eternal principle.

So there has to be something more to writing an opening sentence. There has to be some way to find the opening that is inherent in your story. Some way to boil down your story until the first sentence remains.

So what proven methods are there to derive an opening line from a story?

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This question is not about beginnings, which I have asked about here.