Jack made his announced his own tribe in Chapter 8: Gift for the Darkness. He said "I'm going off by myself. He can catch his own pigs. Anyone who wants to hunt when I do can com too." In the end of he chapter Jack tries to show his power and steal Piggys specs.
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jack takes the hunters and creates their own"tribe".
Jack and two of his tribe (Roger and Maurice) raid Ralph's shelter and steal Piggy's glasses. They do this because they have nothing to light their own fires with and also as an act of power, a deliberate provocation.
Jack leaves for Castle Rock first when he forms his own tribe and has to defend himself from Ralph's tribe.
Jack says that the conch is meaningless on his end of the island. He makes his own rules and does not consider Ralph a leader.
During the first meeting, which was held in chapter 1: The Sound of the Shell, Ralph was elected as the chief. Later in the book Jack formed his own tribe and appointed himself as chief.