how to be a master of a subject?

Study, study and more study.

To become a master of a subject, cram your brain full of information on the subject until your eyes bulge due to the excessive amount of information packed in your head. Never quit thinking about the subject. Go around trying to find new insights into the subject. Make people want to hit you in the head when you talk about it because you talk about it to the exclusion of everything else. When your psychiatrist tells you he is going to try to have you legally committed to an insane asylum if you mention it one more time, go ahead and mention it anyway. Once you are in a straight jacket, talk to the other crazy people about the subject until one of them skewers you in the eyeball with a fork. When you die and you go to heaven, talk to St. Peter about it at the Gates of Heaven until he shakes his head and sneakily flings you into the Fiery Furnace, even though you know you deserve to be in Heaven. When Satan tells you he's going to poke you in the rear with his barbed pitchfork if you talk about the subject one more time, go ahead and talk about it anyway. When Satan exiles you from hell into the Outer Darkness, talk to yourself about the subject in the formless void until you can mentally imagine what you're talking about. Eventually, your thoughts will solidify and become reality for you. You will have your own little universe where you are the master of all you survey. That's how you become a Master of a Subject.

A little practice with everything you are learning never hurts either.

We're all agreed that you need to study your subject and, of course, Smiley has given a light-hearted warning that you should heed anyway or you'll end up ostracised.

I'm going to add another element. You'll NEVER master a subject completely unless you love it.

A friend (Waldorff/John o'Gaunt on Ask) recently referred me to a book on poetry by Steven Fry. It's called The Ode Less Travelled. I was immediately taken by the author's enthusiasm for his subject, by his obvious love of poetry, by the joy he had in sharing that love. It's by far the best book on the subject I've read, and, yeah, I've read a few. The others were all useful from an academic point of view but their authors were writing academically.

To master any subject you're going to need that kind of enthusiasm, that kind of love for what you're doing. Anything less and you might get to be "good" or even "very good" but you'll never make it to the pinnacle of mastery.

Research and study.