I had homework to write a diary as if I was living in the Neolithic period. Can anyone please give me ideas on how to write the diary?

If it was me, I would turn up with a stone on which I had drawn something that looked like a cave painting.

Something on these lines (thank you, Stable Diffusion).

However, I suspect that your teacher is looking for something more literary.

I would recommend the Rudyard Kipling story: The Knife and the Naked Chalk for inspiration.