Raymond Chandler had no fiction writing background when he lost his job as an oil company executive in 1932. He taught himself to write pulp fiction from nothing, starting in his mid forties.

The Direct Answer
He taught himself at 44, after losing his oil company job, by taking a published pulp story apart to see how it worked.
His first sale, Blackmailers Don't Shoot, took five months to write and appeared in Black Mask in December 1933.

Fired at Forty Four

Chandler had been a vice president at the Dabney Oil Syndicate in Los Angeles before he was fired in 1932 for drinking, absenteeism, and affairs with women on the payroll. With the Depression closing off most other options, he turned to the pulp magazines, which paid a fraction of a cent per word but were at least a place a man with no fiction credits could try to sell something.

Rather than guess at how the form worked, Chandler studied it directly. He took a published Erle Stanley Gardner novelette apart to work out its plot mechanics before attempting his own story, an almost clinical way to learn a craft for a man who'd spent the previous decade running an oil company.

Five Months for Eighteen Thousand Words

The first result of that method was slow. Blackmailers Don't Shoot ran about eighteen thousand words and took Chandler five months to finish. He sold it to Black Mask for a hundred and eighty dollars in December 1933. Gardner, working the same market, could turn out a story that length in three or four days and produced roughly a thousand of them across his career. Chandler never tried to match that pace, and the patience is part of why his prose still reads as tightly built decades later.

For how that same painstaking method carried through to the four hour rule and the yellow paper Chandler drafted on for the rest of his career, see the complete Raymond Chandler writing routine.

What You Can Steal