The contours of the narrative are defined by the outline.
Just because something wants to enter from beyond those borders later should not excuse the writer from attempting to put up a starter fence around the questions to be explored, the world to be inhabited, and the characters who move about within.
A gate can still be opened to allow something else in.
Or to let go of an idea, a scene, or a character that is struggling to be happy.
Where to Start Your Outline
How to begin such a task?
A friend has said he simply notices the piece of dust, then watches.
The story coagulates around the piece of dust at the heart of the tale, which, acting as a sponge creates an environment attractive to moisture which quickens the oxidation process.
There also may be "set pieces" in mind as well, key moments which the story wants to move toward. Their attractive force insists that layers be added and steps be made through the plot and through character motion to inevitably arrive.
This dust method distinguishes itself from the seed method. To do that, simply put down a few seeds and see how they grow. (Not just any seeds, mind you.)
A method of outgrowth versus an approach of in-growth.
Types of Outlining
Are you outlining a novel? Why not try Miéville's suggested method (for new writers)?
How about narrative non-fiction? Reedsy's memoir template may work for you.
All of these are just possible ways to get started.
The important thing is to start doing the work of outlining your story.