For most of the next three days, Walter F. Mondale plans to be chewing on a cigar in his Cleveland Park living room, sneakered feet propped on an ottoman, studying videotapes of the stars in Sunday's television extravaganza: himself and President Reagan.

He will read briefing books and, using a yellow legal pad, jot points he wants to make. He will talk strategy and technique with the usual collection of advisers. He will become accustomed to answering questions in 2 1/2 minutes.

And he will hone hard-hitting, one-minute rebuttals to answers served up by Columbia University President Michael Sovern or senior aide Richard Leone, who have been taking turns acting as Reagan to help Mondale prepare for this first of two presidential debates. Sovern was Mondale's law professor at the University of Minnesota.

Mondale, his advisers like to point out, won the right to square off against the acknowledged heavyweight champion of televised political debate by running a gantlet of 57 joint appearances and 13 formal debates in the Democratic presidential primary season.

The advisers say their man is ready -- spoiling, actually -- for the final round. "Mondale has won every televised debate he has been in," campaign chairman James A. Johnson said yesterday after a press briefing.

They scoff at concerns that Reagan has an actor's stage presence while Mondale sometimes seems wooden on televison, and that style, not substance, wins such debates.

"We feel as though he comes across on television as someone who is good-natured, smart and confident," Mondale's press secretary, Maxine Isaacs, said. "If that's what you want from television technique, then he's already mastered it."

Mondale's preparation, she said, may or may not include a full-scale, 90-minute mock debate. The candidate is setting the pace and has not told the staff how he wants to spend the next few days, she said.

So far, it has been informal. "The other night, we were sitting around talking strategy, and he suddenly said, 'Let's try a few questions.' So we did it," Isaacs said.

Voluminous briefing material has sprouted, leading chief speech writer Martin Kaplan to remark, "We've got about half of the Sequoia National Forest in paper."

To help coordinate it, the campaign recruited Lewis Kaden, a New York lawyer, Columbia law professor and friend of Leone.

In free moments during the last several weeks, Mondale has been studying tapes of Reagan and himself in past debates. With video equipment in his living room he can tape his responses to the mock questions between now and Sunday night, then critique them.

Staffers have complained that Mondale has resisted such efforts to analyze elements of his television style, but Kaplan said that Mondale "has grown more intrigued by what you can learn when you watch, and he's had more time to prepare for this one than any of the others."

Johnson called the debate an "extraordinary opportunity" for Mondale to present himself to "more people than have ever seen him before in his career." He predicted that when histories of Mondale's come-from-behind victory over Reagan next month are written, "this debate will be cited as the turning point."

But, in the time-honored tradition of political low-balling, Johnson doused talk of major gains in polls in the days following the debate.

He promised that Mondale would be aggressive and call attention to Reagan's perceived preoccupation with the wealthy, lack of concern for education and the environment and failure to present detailed plans for a second term.

Johnson also cited what he called the "frightening" prospect that the Rev. Jerry Falwell, head of the conservative Moral Majority, might have a hand in selecting new Supreme Court justices if Reagan were reelected. Falwell has made such a reference during the campaign.

Johnson said Mondale would be primed to pounce on any factual mistakes or misstatements that the president might make.