Red Herrings plays two important roles in a mystery novel. One, they heighten the suspense and two, add challenges to solving the mystery by misleading the reader/sleuth.
A writer can create a red herring with characters, the setting, and with objects.
In many classic mysteries, almost every character encountered benefited some way from the crime. Therefore, give them all motives.
Provide a character with the means and opportunity. If a person appears capable of committing a crime but has no motive, there are two possibilities created. The person has a motive that has not been discovered or the person is working with someone who has a strong motive.
Place – Where did the crime happen? Did it happen in the city, country, a small village? During a New Years Eve party? During Mardi Gras?
Date – Was the bank robbed on Friday before a national holiday? If so, who had inside information?
Time – Did the crime happen around a holiday or while the characters were on vacation?
Weather – Was it rainy, snowing or hot outside?
What object appear and what objects do not appear?
What does the detective see at the crime scene? What is not at the crime scene that one might expects to find? Could someone have removed an item? Do all the items belong?
Introduce objects with more than one explanation. Various nuts may be a common item found in a victim’s apartment, unless it is revealed the victim is allergic to nuts. Does the nuts implicate the killer or did the victim have a visitor who brought the nuts with them?
The more ways a reader can interpret an item, better the chance they will make the wrong assumption.
Do not place “red herrings” in a story just to mislead readers. Always have an explanation as to why the objects are in the story. After all, the purpose is to make the mystery more challenging and exciting for the reader.