1. Find a thing you are interested in.

2. Start learning about it and doing stuff with this knowledge. Intermittently look for new information about this thing.

3. Keep on working with this thing. Find ways to use it whenever possible, even if it is not perhaps the most efficient tool for the job at hand.

4. Make mistakes with this thing. Make some little ones, make big ones. Whenever you make mistakes try to ask yourself how to make sure you never make this mistake again. ("Making mistakes" is not a thing you try to do; it is a thing you will do. They can be small. Or they can be stressful and complicated and expensive and scarring. An expert will probably have a few of the latter under their belt, unless they were very lucky and only had a few really, really close calls.)

5. Keep working with the thing. Can you figure out a way to get a multi-year project with the thing going? Do you still fundamentally enjoy working with the thing when you are at the stage when you can do this - and when you are finished with that multi-year project, as well? Can you find a way to work under or with someone who is great with the thing? They'll teach you a lot of stuff that doesn't make it into books, as well as call your attention to really fundamental stuff that you've been blowing off like "kid, the way you hold your tool is gonna fuck your hands up in five years, start doing it this way if you want to be able to keep doing this for the rest of your life, here's some exercises to get used to it". (Seriously: I am an artist and I did not get instructed in How To Properly Hold A Drawing Tool until I was in the animation industry.)

6. Hang out somewhere people ask questions about the thing. Answer them. Answer the heck out of the questions you see allllll the time and are tired of answering and make key expansion shortcuts for them. Play around with the rare, interesting questions and answer them in the most novice-friendly way you can. If you know five ways to solve a novice's question about the thing, then go into some detail on all of these ways and talk about which one you would probably reach for if it came up in practice, and about why that's your choice.

7. Keep on doing the thing. Lots of little projects? Big long projects? Maybe both. It's easier to experiment in the little projects but the big ones are important too.

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There is a character in Zelazny's Chronicles of Amber who is The Absolute Best General And Swordsman In The Entirety Of Reality. He wasn't born that way. Nobody is. But over many centuries of life, he studied and practiced every day. Every day, for hundreds of years, he spent some time studying the great battles of old, practicing his swordsmanship, playing wargames, engaging in actual military campaigns from every level from top to bottom. He has been doing this longer than everyone else alive, and thus, he is an expert.

You do not need to go to this extreme. You probably can't unless you turn out to be an immortal who can hop between alternate realities to see how various changes effect the ultimate outcome of a huge project over multiple trials. But you need to keep coming back to the thing you will become an expert in.

You can't just do one "deep dive" and become an expert. You have to live with the thing, you have to spend time working with it, time just kicking back thinking about the thing, you have to spend time catching up on the latest news about the thing, you have to internalize some of the thing so that the back of your mind just prompts you with little warnings based on something you fucked up ten years ago. You have to dive to the bottom and stay there.

And you will need to sacrifice some other shit on the altar of this expertise. There's only so many hours in the day, and only so many days in your life.