The first time the chant is used is in Chapter 4 after the boys kill the first pig:
Kill the pig. Cut her throat. Spill her blood.
It is repeated at the end of Chapter 4 during the re-enactment of the pig hunt when Maurice pretends to be the pig.
Kill the pig. Cut her throat. Bash her in.
Later it is used when Robert is the pig. The chant changes slightly:
Kill the pig! cut his throat! Kill the pig! Bash him in!
The chant returns to the original version when Jack's tribe gathers in a circle just before Simon's death:
Kill the beast! Cut his throat! Spill his blood.
This chant is repeated four times. Notice the substitution here of beast for pig. It is the beast that they want to hunt and kill now, not a pig. This last time the chant adds another command: "Do him in!" This addition echoes the Lord of the Flies ultimatum to Simon, ordering him to "play" or else others will "do him." We see here what "do him" actually means.
I like this question because analysis of the words shows a growing savagery on the island and ties together hunting and the pig re-enactments ultimately resulting in Simon's death. It would make an interesting essay.