How to Write a Poem That Explores the Theme of Nostalgia?

I wondered if anyone could direct me to a good resource for exploring the theme of nostalgia in a poem? I am trying to write a poem for school, and the teacher wants us to explore a particular theme. I am choosing nostalgia. Are there any particular things that I should focus on when writing a poem about nostalgia? I would love to hear your tips for writing a poem with this theme.