This passage from “Little Gidding,” the fourth of T.S. Eliot’s “Four Quartets,” describes the process as well as anything I have ever read.
What we call the beginning is often the end
And to make an end is to make a beginning.
The end is where we start from. And every phrase
And sentence that is right (where every word is at home,
Taking its place to support the others,
The word neither diffident nor ostentatious,
An easy commerce of the old and the new,
The common word exact without vulgarity,
The formal word precise but not pedantic,
The complete consort dancing together)
Every phrase and every sentence is an end and a beginning,
Every poem an epitaph. And any action
Is a step to the block, to the fire, down the sea’s throat
Or to an illegible stone: and that is where we start.