Do it everyday. Have your livelihood (main income) tied to it. And I assure you that there is no way you won't be extremely good at it.
If you are passionate about something, maybe it's boxing, swimming, karate, singing, music or writing, and you are worried that you are not extremely good at it. Then you just have to ask yourself a question: how badly do you want to be extremely good at it? And if you answer is very badly, then you should take up a part time job in it. Be an assistant boxing coach, be an assistant lifeguard, join school of music, write everyday...
Mastery comes from practice and the height of practice is to turn it to your practice (job).
Today, I am extremely good at Excel and Business Data Analysis, and I can tell you that the main reason is because I do it everyday and have my livelihood tied to it. It doesn't matter what mood I am in or whether I feel motivated or not, there is always a pending job to complete if I want to have money in my account. So I end up doing all kinds of jobs. Even projects I never imagined anyone would want to do with Excel. Every month, I am newly impressed by how much Excel can do and notice myself getting more extremely good.
The only downside is that you can't be extremely good at a lot of things. You will have to pick what's most important or lucrative. No matter how much you love watching wrestling, it would be a poor choice to pick wrestling as what you want to be extremely good at. Ever wondered why in those movies about world-best assassins and unbeatable fighters, even the ones that turn to international crime to monetize their skills; they still don't make as much money as the guy with a head for business. And they often have a damaged/broken life. Then let's bring this closer to home. Think Nigerian policeman, mobile policeman, soldier, navy and air forceman. Even the most greedy/corrupt/bullying among them are not as rich as their counterparts in other government jobs. And the reason is because there is a big limit to how much you can extort a living from intimidating others or brawling. So it might be more effective to not be a force man so you can have enough money to hire forceman/forcemen to do whatever you want.
Maybe that's not a very good example but you get the message. Pick a passion that's got huge potential, where the average is cool. Not some passion that you'll have to be way above average to pull a decent influence. Another example to avoid is stand-up comedy.
Once you've picked the right passion, then begin to transition to making a part time earning from doing it almost everyday. And you can be sure that you'll be extremely good at it.