The mysterious postcards would arrive randomly in the mail, tucked in envelopes addressed to Doug Parcells.

The same Florida State mascot would stare at him each time from the front of the postcard. The back would remain nearly blank.

The anonymous author offered no greeting. No signature. He just scribbled a sentence or two in the empty space, a message to get Doug’s attention.

If you’re fat, slow and lazy, don’t play football. Get a job at the Dairy Queen, read one.

Barbells and sandwiches don’t mix, read another.

Before leading the Giants to two Super Bowl championships, before becoming the first NFL coach to take four franchises to the playoffs, Bill Parcells was a no-name college assistant in the early 1970s still honing his motivational techniques in Tallahassee, Fla.

And his baby brother Doug, a once-pudgy football player headed to Virginia, was his first project.

“He was trying to help me get ready for college football,” Doug Parcells said. “It would just be that postcard with a message.”

No one knew then just how far Parcells would go, how hard he would push, how many insults and pointed one-liners he would fire at his players to motivate them.

Now we know the answer: Canton, Ohio.

The self-described “Jersey Guy” from Hasbrouck Heights and Oradell was inducted Saturday into the Pro Football Hall of Fame, surrounded by family, friends and a horde of his adoring former players.

On the journey to Canton, Parcells, 71, went to three Super Bowls. He coached the Jets to the 1998 AFC title game two years after they suffered a 1-15 season. He won 183 games, 10th-most in NFL history.

And he launched one of the league’s most accomplished coaching trees, one that includes Bill Belichick, Tom Coughlin, Sean Payton and eight Super Bowl victories.

“Hey, we won two Super Bowls so clearly the man can coach. But it was his style,” Giants great Lawrence Taylor said in a statement through his agent, Mark Lepselter.

“He always had a sense (of) the vibe in the locker room and he always knew which buttons to push.”

Parcells (183-138-1 in 19 seasons) was a “master motivator.” A brilliant strategist. And the man who helped turn Taylor, Harry Carson and Curtis Martin into Hall of Famers, and Phil Simms, Joe Morris and Mo Lewis into Pro Bowl picks.

“I’ve been around a lot of different coaches, some of the great ones,” said former Giants kicker Matt Bahr, who played for Joe Paterno, Chuck Noll and Bill Walsh, “and I wouldn’t hesitate to say that there may be his equal but there’s none better.”

Not bad for a guy who almost was fired after his first season as an NFL head coach in 1983, when the Giants went 3-12-1.

Three years later he led his hometown team to a championship. He did it again in 1990 in a dramatic Super Bowl.

But before Parcells coached the Giants, Patriots, Jets and Cowboys, he was a fiery, hard-headed athlete named Duane.

“The first time I met him, he kept looking over at me as we’re running through the drills, checking me out to see if I met his approval,” said Mickey Corcoran, 91, Parcells’ basketball coach at River Dell, N.J., and lifelong mentor.

Parcells was 15 at the time.

STRESS AND MISERY

They all have a story.

It usually begins with an expletive. And it always ends with Parcells as the villain.

“I played under him for eight years. I had cold sores for eight years because it was constant stress,” Simms said.

“It was miserable to play for him,” former Giants center Bart Oates said. “Listen, if you wanted to go enjoy and have fun, Parcells wasn’t the guy.”

He yelled. He argued. He pushed players past their limits.

And when Parcells wasn’t threatening jobs, he used a blistering wit to send messages, often with the entire team within earshot.

“He was able to get into the psyche of the players,” Carson said. “There were some guys who would say, ‘You either hated him or you loved him.’ Sometimes you felt both within a five-minute time frame.”

“I viewed myself as a teacher,” Parcells said last month. “I grew up in a family that was confrontational so I think I carried some of that forward with me.”

But he also is the kind of guy who couldn’t attend his 50th high school reunion yet picked up the tab for everyone who did.

The kind of guy who will find many of his former players in Canton this weekend just to celebrate him.

“Bill will be defined by the Super Bowls that he’s won but also the impact that he’s had on players,” Carson said.

JERSEY GUY

Doug Parcells stood in the driveway of his boyhood home in Oradell, N.J., on a recent summer morning, pointing out the reminders of his famous brother.

There sits the second-floor window Bill used to climb into, having had to scale the garage roof to avoid his parents when he snuck out at night.

And there you can almost see the batter’s box Bill drew on the foundation, where he taught then-7-year-old Doug to hit left-handed because he was “porky.”

“He told me, ‘Well, now you’re a step and a half closer to first base. You’re going to need that because you’re slow,’ ” Doug said of Bill, who’s 12 years older.

A few years before, Parcells had ditched his given name of Duane. He never took to it and after he was confused with another boy named Bill, he became Bill, too.

Then Parcells became a three-sport captain in River Dell’s inaugural graduating class of 1959.

He was the quarterback. A pitcher. And the best player on the basketball team.

“He was a leader just like he is now,” said Chuck May, a boyhood friend.

Corcoran remembered a young man so competitive that “sometimes he might lose his composure.” They even clashed at times, times when Corcoran had to impart lessons.

Parcells would use many of those lessons in the NFL.

But he had to sacrifice plenty to reach the pinnacle of his profession.

His health. Time with his family, especially his three daughters. He even coached a Cowboys game hours after attending the 2005 funeral of his brother, Don.

“He gave it everything,” Oates said. “He sacrificed everything to be successful.”

Corcoran saw the same traits when Parcells was just a 15-year-old basketball player.

Corcoran once sat with his brooding star in a postgame locker room after a narrow loss that would have been a blowout if not for Parcells.

He tried to lift his spirits, but the sophomore would have none of it.

“Coach, either you win or you lose. And we lost,” Corcoran remembers him saying.

“And then he trudged off into the night.”