A first person narrator must have their own voice. It must be an individual voice, differing from your own. You need to immerse yourself in the character to capture their attitude, their mannerisms, their thoughts and their memories. They are unreliable narrators in the fact that we only get to see their side of things. Their memories may be faulty, their thoughts narrow and close minded, but we live and breathe through them.
You have to maintain a balance in your narration. If the narrator becomes too emotional, they run the risk of being melodramatic; too graphic and it's too intense; too cold and calculated they become too distant. Think of how Watson narrated Sherlock Holmes' adventures. Where we might have despised Holmes for his calculated thinking and his unemotional state, we can admire the man and welcome the logic with which he solved his cases while still seeing the human side of him through the more emotional Dr. Watson.
One of the problems with first person POV is the distance in time from the events. Usually your narrator is telling the story from some distance, as the events are in the past. But distance in time can lead to faulty memories and the story becomes skewed as the narrator tells things from their unreliable thoughts.
If you're going to make your narrator unreliable and a liar, show us. Let us catch them in a lie. If you're going to make them cruel, don't tell us. Show us. Let us see how they justify their actions. If you're going to have them do something heroic, make it so they don't realize how heroic they're being. If they boast about it, it makes them distinctively unlikable.
A really good example of the first person narration that I've personally read is the Hunger Games trilogy. Our focus narrows to Katniss and her world. Her thoughts, her emotions, her beliefs color our perception of the world. Her sensory descriptions help us live through her eyes.