How did he shave points?

Its really the same in any other sport, he just didnt play as hard as he could. He may have overthrown a few receivers on purpose, or fumbled the ball on purpose. Basically just not playing up to the level that he should be playing at.

To put it as best as I can.
Lets say it's Peyton Manning instead. Manning and the Colts are favored by 10 over the Jets. The Colts are leading by 14, and gamblers tell Manning he has to shave points. So he makes mistakes on purpose, and next thing you know, the Colts win by 7, because of what Manning deliberately did.
The league finds out. Bans Manning for life, for being involved with gamblers.
That basically is point shaving.

I will give you a real life example;

In 1919 the chicago white sox shaved points; they were in the championship series and lost games on purpose due to bribes from gamblers. They were caught and banned for life. The revelation that the particular world series was fixed due to gambling almost killed the sport of baseball and still to this day there is zero tolerance for gambling.

In the game in the film, Crewe admits that he owed loan sharks money and his deal was to shave points (ensure his team would not win by a certain score or lose entirely) to settle the debt.

In the mean machine/guards game, the warden again threatens Crewe to shave points (lose by 2 touchdowns) or else be charged for caretakers murder. He does do this; throws an intentional interception and fumbles and then fakes an injury.