You can start by looking on fanfiction websites like Archive of Our Own. They usually have a wide range of fanfictions, including ones about Nana Shimura.
One popular 'nana x nana fanfic' story could be about their adventures in a new city together. They might discover new music and bond over it, exploring the local music scene.
First, understand the characters well. Know their personalities, dreams, and fears. For the 'nana x nana' pairing, it's important to capture their unique bond.