SOUTH SAN FRANCISCO - Paul Williams, a distinguished-looking person of middle age who could pass for a teacher or businessman, described how he became what he really is - a science fiction writer - at a writers' conference earlier this month.

``I started writing science fiction because I hurt my back,'' he said. Williams, a teacher, said he could not bear to watch TV while he was recuperating from his back injury. ``I found TV insufferable,'' he said.

Instead he spent that summer reading science fiction novels at a rate of one to two a day.

``I read 200 novels before I began my own - I knew the genre,'' he said.

He took advantage of the knowledge he picked up during that period of enforced rest to prepare a course on the major themes in science fiction. He didn't know yet that he would be writing his own sci-fi novel. He also carried a notebook with him at all times - a habit developed in college - to jot down ideas.

``I get an idea and I save it on paper,'' he said.

Early one July day the next summer Williams got an idea for a certain fictional scene. He began writing it down in a hurry and became totally engrossed in the story. Before he quit, he'd typed up the first three pages of his first novel, which was ``The Breaking of Northwall'' (1981). This became Book One of the seven-volume Pelbar Cycle.

Williams said he later tried to guess where the idea that became ``Northwall'' came from. He traced it back to a book read years and years earlier about the Lewis and Clark expedition through the western United States.

``I had a lot of free time and wrote every day,'' said Williams. ``I never let a day pass without writing. By early September I'd finished my novel.''

He follows the advice of other writers who commit themselves to putting down on paper 1,000 words a day. This kind of discipline is absolutely necessary for undisciplined writers, he says.

``(Without discipline) you find that six months have gone by and you haven't written anything,'' said Williams.

Williams sent his manuscript to Del Ray books and heard nothing for nine months.

``Then I got a phone call saying that they wanted to buy it and that night I began a sequel that turned into a seven-part series,'' he said.

Williams says science fiction is one of the most vibrant publishing fields. New writers are welcomed. It is not a pretentious genre. Readers, he said, like plain-spoken stories with way-out subject matter.

``Science fiction is written in the ancient ways,'' he said. By that he means that good science fiction writing adheres to traditional forms of story-telling, with a beginning, middle and end.

``No plotless stories,'' instructs Williams.

He says the line between science fiction and fantasy is not always clear but essentially fantasy makes up its own rules (horses fly) while sci-fi adheres to some rules of science.

The amount of science in science fiction need not be substantial.

``Sometimes only one scientific fact is necessary,'' Williams said. And the plot can involve a society or character living in opposition to that scientific fact.

The most popular plots in science fiction involve the post-holocaust story, alien encounters, time travel, space travel, cyborgs, artificial intelligence, politics on a galactic scale, environmental problems, mortality and immortality, plague and viruses, nuclear and radioactive disasters, and instant communication or lack thereof.

``Ultimately, all these plots require good characters doing things like people do or ... creatures acting like people do,'' he said.

And if you are willing to drag yourself away from your computer screen, you can meet a lot of characters in real life. Williams recommends attendance at one of the increasingly popular science fiction/fantasy conventions.

Fans of science fiction have their own clubs, their own magazines, their own conventions, their own worlds. These worlds are recreated just about every weekend at sci-fi conventions. The magazine Locus has listings of these conventions, he said.

``Go to one just for the fun of it,'' he recommended. The conventions offer serious panels, light-hearted talks, 24-hour film festivals, electronic games, role-playing games, art shows and dealer rooms specializing in memorabilia.

``These function as quasi-churches for a lot of people,'' said Williams. ``For the writer, this has a down side. There can be too much glitz and not enough time centered on writing.''