1. Your protagonist should not be in any sort of profession where it would make sense for them to be solving mysteries. This means they don't require warrants, or need to file a report for every bullet fired from their gun.
2. There should be a secondary protagonist who is law enforcement, ex-law enforcement, or a P.I. There relationship with the protagonist may be platonic or otherwise.
3. Only and idiot would turn down such an obvious chance for sexual tension. "Platonic" is a bad word that will now bin and never think of again.
4. The protagonist is pursuing the case because they are either the main suspect, and don't trust the coppers to do their job, or their friend or family member is the main suspect...who doesn't trust the coppers to do their job.
5. Your protagonist is single and lives alone, with the exception of their pet cat, dog, or tortoise.
6. Your antagonist, preferably a serial killer, was once intent on being immaculate and getting away scot free, but has now thrown their panties to the wind in an irrational and egotistic ploy to gain the protagonist's attention.
7. The protagonist keeps information from the police because they are paranoid. "Paranoid" in this context is a synonym for "stupid."
8. If your protagonist is in trouble, they should not dial the emergency number, but instead call the cop/ex-cop/P.I. and leave a voice mail.
9. The protagonist figures out who the killer is. They go to confront the villain, making sure they leave a handy voice mail first, and are then attacked.
10. After a climactic scrape with the villain, the protagonist is knocked unconscious, and the cop/ex-cop/P.I./thing-a-ma-bob swoops in and takes them to the hospital, where they then wake and are clued in on all the missing details. The killer has either confessed or met their demise. I tease! They're dead.
Come along, there's convenience afoot.