An unreliable narrator is a protagonist who can’t be trusted to tell the events accurately. By having this narrator be insane, evil, delusional ect. hooks the reader to find their own truth.
Writing an unreliable narrator allows the author to play with withholding the truth and when to reveal it, keeping the reader on their toes.
If you would like to write your next story with an unreliable narrator in mind follow these tips:
Withhold information. Readers are used to having more information than the characters. So play with the idea of withholding information, and what information you would withhold.
Hint at the qualities early on. More often than not your narrator will be unreliable from the start. Unless it is a straight they suddenly developed. However, in order to make a fluent story your narrator should be unreliable from the start.
Characters interpretation. You don’t need a ton of other characters’ perspectives. However, you should use them to reflect the inconsistencies in your story.
Even if you don’t want your narrator to be completely unreliable you can experiment with vulnerability. Even Harry Potter a gives the reader misinformation in Prisoner of Azkaban when he thinks the escaped prisoner who killed his parents is actually a lie. A slight flaw can make the narrator seem more believable.