How can someone stop atrial fibrillation?

Depends: New onset atrial fibrillation is often self limited, going away by itself. But the cause dictates the subsequent course. Thus treating things like high blood pressure, heart valve disease, heart failure, over active thyroid hormones, coronary disease, and avoiding alcohol may be important. Some drugs are useful to maintain rate and rhythm. If needed, electric shock (cardioversion) is effective.

You can't: If it's new, it's usually self-limiting. If it doesn't stop on it's own, there's nothing you can do, but your doctor can give you medication that will convert your rhythm and if that fails, it can be electrically converted while you're anesthetized. Note that if you have certain risk factors, you should have your blood thinned prior to any attempt to convert the rhythm.

Reduce risks: A fib will be more likely in setting of excessive stimulation of the sympathetic nervous system. Alcohol use results in hyperstimulation of sympathetic nerves with Adrenalin during the process of alcohol metabolism. Other factors--pain/fever/breathing problems/sleep apnea can stimulate a fib episodes too.