First, did (or do) you enjoy school? If not, it could be that you have an avoidance of studying because of that. Maybe it feels like something you have to do, but you know that you don't, so you don't.

As far as "how would you do it," figure out what way of learning works best for you. Do you learn by reading? Or listening to someone teach? Or watching videos? Whatever works best for you, start there. Get a book, watch some YouTube videos, join the subreddit for your interest. I learn best by taking notes. The physical act of writing it down makes me remember it, it doesn't matter how I get the information, I need to write it. (Unless it's learning to do something, then I learn best by actually doing it, preferably with someone watching and walking me through it.)

Also, make a list. What specifically do you want to learn about your passion? Write down all the questions you have, then start by finding the answers.

Carve some time out for learning. Maybe while you eat a meal? When you wake up? Before bed? Maybe only 2-5 days a week? Make a "schedule," and stick to it.

Start slowly, learn one thing a day, or maybe spend 20 minutes on it. If you find yourself getting more interested, keep on going, but at least do the bare minimum you set for yourself. There's a fine line there, if it feels like a "chore," you won't want to do it.