Start a career that somehow involves writing.
With two exceptions, every author in the group whose job we were able to find out does work that involved use of their verbal skills in some way. Some work in fields rather remote from creative writing: several are lawyers, and Chesbro was a special ed teacher. But the most heavily-represented profession is teaching writing or literature. (See chart.) Most of this group of authors seem to have chosen to earn a living doing what they're best at and most interested in.
It is very rare for even much-published novelists to make a living solely from their fiction.Most of this group had to work at their jobs until retirement age. Even the prolific and successful Brooks did not become a full-time writer until he was 40. The almost-equally prolific Chesbro occasionally had to take temporary work to supplement his writing income. Jackson, Townley, and Zaroulis appear to have been, for long periods of their lives, full-time writers, though it is unclear whether their writing brought in enough money to live on.
In my case (Linzee), I have held full-time jobs for the last several years. In earlier years, when writing and publishing books was my first priority, I had a variety of part-time jobs, and it stilll wasn't enough to live on. I heartily endorse the statement of veteran mystery-writer John Lutz: "The writer's biggest advantage is a spouse with a steady job."