Rule 5 of 24 · Chapter I — The Laws Beneath the Story
Write the rules down before you break them
Why this rule exists
A world carried entirely in your head will contradict itself, not because you are careless but because memory is a liar that smooths over inconvenient facts. The distance between two cities, the year of the old war, the exact price magic exacts, all of these drift unless they are written where you can be held to them. A rule you have recorded is a rule you can reason with, extend, and deliberately violate for effect; a rule you merely remember is a rule you will accidentally betray on a tired afternoon in chapter eleven. The notebook is not the world, but it is the fence that keeps the world honest.
In practice
Keep a plain reference, a document, a wiki, a stack of index cards, whatever you will actually maintain, and put in it every hard fact and firm rule the moment it hits the page. Record what you have committed to, not every daydream: the things a reader could later catch you contradicting. Note the limits especially, since those are the ones stories love to quietly erode. Consult it before you introduce anything new, and when you choose to break a rule for a climax, break the written one on purpose, knowing exactly what you are spending. The goal is not a beautiful bible but a trustworthy one.
When it doesn't apply
Do not let the archive become the work; some writers world-build for years and never finish a story, mistaking the notebook for the novel. Much can also stay usefully undecided until a scene forces the question. Write down what you have shown or will need to keep straight, and leave the rest free.