In chapter 1 of Things Fall Apart, how does Achebe foreshadow Ikemefuna's presence?

I am not too sure if "foreshadow" is the correct word to use here. Let us remember that foreshadowing is when the author plants hints or clues about what is going to happen later on in the novel. A good example would be the numerous times that Okonkwo expresses his rage and anger, which clearly foreshadows the murder that he commits later on. However, Chapter One doesn't really foreshadow the arrival of Ikemefuna into Okonkwo's family, it just tells us that this is going to happen. Consider what it says at the end of the chapter:

And that was how he came to look after the doomed lad who was sacrificed to the village of Umuofia by their neighbours to avoid war and bloodshed. The ill-fated lad was called Ikemefuna.

Thus we can see that there are no hints or subtle suggestions that this is going to happen. The text rather states it openly. Therefore I would argue that "foreshadow" is not the correct word for this question. Having stated a brief history of Okonkwo, which explains how he managed to be successful in spite of the shortcomings of his father and his upbringing, the narrator concludes the chapter by explaining how Okonkwo's position in the tribe led him to be responsible for Ikemefuna.

How does Achebe foreshadow Ikemefuna's ultimate fate in Things Fall Apart?

Ikemefuna's fate is one of the most difficult parts of Achebe's novel. It's not much of a mystery that Ikemefuna will come to a bad end: he's introduced as the "doomed lad" who was given to Umuofia explicitly as a sacrifice. Okonkwo's role in Ikemefuna's death, however, is not clear, but is foreshadowed by the narrator's assessment of Okonkwo's character in the paragraphs immediately following Ikemefuna's coming to live with Okonkwo. Okonkwo hates everything his father loved, especially "gentleness and idleness." It's clear that Okonkwo's fear of being seen as "weak" will cause him to act against the boy, and, for the boy's part, "he could not understand what was happening to him or what he had done."

Okonkwo's growing regard for Ikemefuna, Ikmefuna's friendship with Okonkwo's son Nwoye, and Ikemefuna's tendency to refer to Okonkwo as "father" all point to a reckoning, since we know that Okonkwo eventually will not be able to abide by the tenderness he feels for the boy. In fact, it is because Okonkwo has become the boy's father figure that he is told to not participate in his killing, but it is this prohibition that naturally causes Okonkwo to kill him as a show of strength.

Even though Ikemefuna is beginning to feel comfortable living with Okonkwo, he is a sacrificial offering from the time he's given to Okonkwo. Achebe refers to Ikemefuna as a "doomed, ill-fated lad" since he has been given to Umuofia to atone for murder.

Some critics have also seen the event when Okonkwo breaks the peace during the Week of Peace as foreshadowing Ikemefuna's death. Okonkwo beats his youngest wife, and the priest has Okonkwo sacrifice a goat and a hen as part of his repentance. His willingness to do this might foreshadow Okonkwo's willingness to kill Ikemefuna.

How does Ikemefuna's death foreshadow Okonkwo's fall in Things Fall Apart?

Ikemefuna had been placed in Okonkwo's care after it had been decided that another village, Mbaino, had to offer up a young virgin and a young boy as compensation for the killing of Ogbuefi Ujo's wife when she went to market there. Ikemefuna, who was fifteen at the time, lived in Okonkwo's care for three years before it was decided that he had to be executed.

Ezeudu had asked Okonkwo not to be involved in Ikemefuna's execution, but when the men came to take Ikemefuna, Okonkwo accompanied them. When they were far outside of the village, one of the men struck Ikemefuna, but the blow did not kill him. Ikemefuna cried out, "My father, they have killed me!" and Okonkwo, out of fear, struck him down. He was afraid that the men would think him a coward if he did not do so.

This fear is born out of Okonkwo's pride. He always had to prove himself a man and his harsh actions towards his family and others spoke volumes of what it meant for Okonkwo to be a man.

It is in this sense then, that Ikemefuna's killing foreshadows Okonkwo's fall, for it was this same fear, borne out of pride, that led to Okonkwo killing the  court messenger who came to break up a meeting in their village.

The gathering had been called to discuss what kind of action was to be taken against the evil which had invaded their society - this is, of course, a reference to the missionaries. Okonkwo had been a the forefront of protest and they had burned down Enoch's compound and the church. The village had to pay 250 bags of cowries for their men, Okonkwo included, to be released from captivity.

After the messenger's killing, the Commissioner and a band of soldiers came to arrest Okonwo.

Here again, it was Okonkwo's pride which drove him to suicide. Instead of succumbing to the will of the Commissioner and his men, Okonkwo hanged himself, in spite of his action being deemed an abomination by his clan, who would refuse to bury his body or even touch it.