To start an 'Into the Spider - Verse fanfiction', begin by brainstorming ideas related to the multiverse concept. You could jot down different dimensions you want to explore or new powers for the characters. Then, establish the tone of your story. Do you want it to be a light - hearted adventure or a darker, more serious take on the Spider - Verse? After that, start writing a short introduction that grabs the reader's attention. Maybe start with a really exciting action scene involving your favorite Spider - Person. As you write, keep in mind the unique personalities of each character from the movie and build on them in your story.