What steps can someone take to become a full-time writer without teaching or working from home, and with no other commitments or responsibilities?

You can manage some spare times to write. Try to make it everyday at least to write 100 / 250 words. Make it a streaks. Think before writing what you want to write . For the convenience you can carry a mini sized laptop always with you. By this you can write from anywhere at anytime whenever some ideas tickle on your head.