On devient bon écrivain comme on devient bon menuisier: en rabotant ses phrases.
Notes from Wikiquote
As quoted in Anatole France en pantoufles by Jean-Jacques Brousson (1924); published in English as Anatole France Himself: A Boswellian Record by His Secretary, Jean-Jacques Brousson (1925), trans. John Pollock, p. 85
Variants
You become a good writer just as you become a good carpenter: by planing down your sentences.