In the second volume of his Johnsonian Miscellanies (Clarendon Press, 1897), George Birkbeck collects the “Anecdotes and Remarks” of Thomas Percy (1729-1811), the Bishop of Dromore, best known for his Reliques of Ancient English Poetry (1765). Percy describes Johnson’s mode of composition, which is a variation on what I have described. I don’t think we need to take Percy’s medical explanation of Johnson’s practice too seriously:
“Johnson’s manner of composing has not been rightly understood. He was so extremely short-sighted, from the defect in his eyes, that writing was inconvenient to him; for whenever he wrote, he was obliged to hold the paper close to his face. He, therefore, never composed what we call a foul draft [first draft] on paper of anything he published, but used to revolve the subject in his mind, and turn and form every period, till he had brought the whole to the highest correctness and the most perfect arrangement.”
Writing is highly idiosyncratic. Such things can’t be taught. It’s too much the expression of sensibility, which is another name for style, so laying down the law is futile. My stuff is partially written before I commit it to print, but still a long way from “the most perfect arrangement.” But I share at least one quality with Johnson, as described by Percy: “The writer of this note has often heard him humming and forming periods, in low whispers to himself, when shallow observers thought he was muttering prayers, &c.” All the words you’ve just finished reading, mine and Percy’s, I’ve hummed and whispered as I typed.