How does William Golding use the conch to depict the descent of the boys into savagery? For example, how does Jack break the rules of the conch?

In Lord of the Flies, the conch is a powerful symbol of civilized society. Readers can track the disintegration of the boys' civilization and their descent into savagery by following the way the conch is treated. 
At first, the conch represents order and following the rules. When Ralph blows the conch, it confers authority on him, and the boys agree to "let him be chief with the trumpet-thing." Jack makes his first challenge against the conch when Piggy scolds the boys for making an inefficient fire. Jack says, "The conch doesn't count on top of the mountain." Ralph asserts his authority, claiming that "where the conch is, that's a meeting." The boys agree, and Jack falls in line, saying, "We're not savages. ... We've got to do the right things."
In chapter 5 when the evening meeting deteriorates around talk of beasts and ghosts, Jack uses the fear to lead the boys away from the meeting. Piggy urges Ralph to use the conch to call the boys back, but Ralph senses that they would not respond. If all the boys outwardly defied the conch, that would be the end of their ordered civilization.
In chapter 6 when Samneric report on seeing the beast on the mountain, Jack becomes openly defiant against the conch:

"Conch! Conch!" shouted Jack, "we don't need the conch anymore. ... It's time some people knew they've got to keep quiet and leave the deciding things to the rest of us."

Jack is advocating a totalitarian type of governance as opposed to Ralph's democratic regime. Ralph silences Jack for speaking without holding the conch, and the boys stay on Ralph's side.
In chapter 8, Jack tries once more to swing the boys to his side by proposing a vote of no confidence toward Ralph. Jack holds the conch, but when he loses the vote, he lays the conch carefully on the grass and goes off by himself. When some of the boys join him, he tells them, "I'm going to get more of the biguns away from the conch and all that." He has rejected the ordered society that the conch represents and wants a society based on his own supremacy. When the boys raid Ralph's camp for Piggy's glasses, Piggy fears they are coming for the conch. But that is because he misunderstands what Jack values. Jack no longer values the type of society the conch represents.
Finally, when Ralph, Piggy, and Samneric confront Jack at Castle Rock, Piggy insists they take the conch. Jack responds with violence, sparring with Ralph with a spear and kidnapping Samneric by force. Then the boys begin to throw rocks and Roger dislodges the boulder, which murders Piggy and shatters the conch. This represents the final descent of the boys into savagery; after that, they deliberately hunt Ralph with the intent to kill him.
Following the role of the conch, particularly the way Jack views it, helps readers track the descent of the boys' civilization from order to savagery.