How Do You Quit Nail Biting?

A study found that people who bite their nails are more likely to calm down when they're under stimulated. The nicotine in cigarettes can sort of be a stimulant when you are under stimulated, and it can be a accent when you are over stimulated. How do you quit? If if you're a nail biter and you want to stop? I've read several ideas. One is just trying to replace nail biting with an incompatible alternative activity.