How are the seven deadly sins represented in the novel?
Very quickly with an example of each:
Pride - Almost all of the characters at some point. Jack when he isn't able to kill the pig the first time; Ralph when people aren't following his directions; Piggy when he knows what to do but no one will listen to him.
Envy - Certainly Jack when Ralph is elected leader. Probably also Piggy wishing he fit in more like Ralph. Later, Ralph wishing he possessed some of the carefree qualities of Jack.
Anger - Jack throughout most of the novel, particularily when he senses the other boys questioning his hunting ability after not killing a pig.
Greed - Jack is very power hungry.
Sloth - Piggy's character is represented by a sloth-ful attitude prior to arrival on the island. While there, he is a poor worker also. There are other examples also such as when Ralph and Simon are left to make shelters on their own while the rest of the boys have fun.
Gluttony - After the slaughtering of the fat momma pig the boys have a huge feast where they can finally eat their fill. This indulgence leads to the crazy atmosphere that eventually kills Simon.
Lust - Depends on where you want to go with this. You could say that Ralph is lustful for a community of civilized boys that act proper and dignified. Or you could take the Freudian route and use the scene where the pig is stuck with a spear in a very unfortunate place.
There are more examples, but that should get you started.