To start writing detective fanfiction, it's important to do some research on detective work. You can read real - life detective stories or watch detective shows. This will give you an idea of how detectives approach cases, what kind of evidence they look for, and how they interrogate suspects. Also, don't forget to add some twists and turns to your story to keep the readers on their toes.
First, come up with a unique detective character. This could be someone with an interesting backstory, like a detective who was once a criminal but now uses their knowledge of the underworld to solve cases. Then, think of a mystery. It could be a murder, a disappearance, or a theft.
You need to establish the setting clearly. Is it a big, bustling city like New York or a small, sleepy town? The setting can greatly influence the type of crimes and the way the detective solves them. For example, in a small town, secrets are harder to keep and everyone might be a suspect. After that, start writing the plot, with the detective gradually uncovering clues.