How does Ralph learn of Jack's plans for him?
In chapter 11, Samneric were captured by Roger at Jack's command. They were forced to join Jack's tribe and acknowledge him as chief. On Castle Rock Jack threw his spear at Ralph "with full intention," wounding Ralph. Piggy had been deliberately murdered before Ralph's very eyes.
In chapter 13, Ralph is alone, an outcast. He sneaks up to the top of the mountain in the dark when he realizes that Samneric are on guard. He has a whispered conversation with them, and they warn Ralph to go away, explaining, "They hate you, Ralph. They're going to do you." They tell Ralph that Jack plans to hunt him the following day, having devised a methodical canvassing of the island to make sure they find him.
Ralph asks several times what they will do with him when they catch him. The twins warn that both Roger and Jack are "terrors," but cannot bring themselves to say specifically what their ultimate goal is. Finally they say, "Roger sharpened a stick at both ends." Ralph contemplates the meaning of that phrase and tries to persuade himself that the boys are still boys. This creates dramatic irony, for the reader knows exactly what the stick sharpened at both ends means. Roger had taken special delight in shoving the stick into an opening in the sow's body and driving it in until the sow expired. Roger and Jack have the same end in mind for Ralph. The next day, Ralph finds himself fighting and running for his life, and by then he certainly knows that he will never be able to walk up to Jack and say, "I've got pax," and "pretend they were still boys."
The twins, Samneric tell him that Jack has ordered a stick to be sharpened at both ends. They also allow Ralph to get ahead in the hunt since they spot him and choose not to yell out for the other boys.