He asks him to sharpen it at both ends with intent of putting one end in the ground and the other end to have Ralphs head on it. This is not explicitly stated in the novel but we know this because of the Lord of the Flies
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The pigs head, which was offered to the beast, was mounted on a spear which had been sharpened at both ends. One point to hold the head and one point to stick into the ground. The implication is that Jack intends to behead Ralph and then mount his head on a stick as an offering to the beast.
They warn Ralph that Jack plans to hunt him the next day and that Roger has "sharpened a stick at both ends."
It seems that Roger has become the unofficial second in command to Jack and also chief torturer.
It seems to mean that Jack's presence was drawing out the evil in Roger.
Ralph, Jack, and Roger are very frightened when they finally see the beast.