Raymond Chandler's clearest statement on writing advice appears in his 1944 essay The Simple Art of Murder, a defense of detective fiction as a serious literary form.

The Direct Answer
Treat genre fiction with the same sentence level seriousness as literary fiction.
Chandler made the case in his essay The Simple Art of Murder, still one of the most quoted defenses of crime fiction ever written.

No Dull Subjects, Only Dull Minds

Chandler's essay, published in The Atlantic Monthly in December 1944, argued that detective fiction had been dismissed by critics who mistook the genre for its worst examples.

Everything written with vitality expresses that vitality: there are no dull subjects, only dull minds.

Raymond Chandler, The Simple Art of Murder, 1944

That line is as much a description of his own method as a piece of criticism. Chandler thought most crime writers simply weren't bothering to write one sentence at a time with any real care, not that the genre itself was beneath serious craft, and he held himself to a different standard, one built out of the same patience he'd used to teach himself the form a decade earlier.

The Detective as a Standard, Not a Character

The essay's most quoted passage describes the kind of man who must go down mean streets and is himself neither tarnished nor afraid, a man of honor by instinct rather than announcement. It's easy to lift that line as a description of tough guy heroism, but in context it reads more like a craft standard Chandler was holding his own prose to: a sentence had to earn whatever it claimed, without slipping into anything overwritten or sentimental.

For how this philosophy connects to the four hour rule and the yellow half sheets Chandler drafted on, see the complete Raymond Chandler writing routine.

What You Can Steal