How do you study and remember what you learned?

I'm turning 30 this month, and I was just diagnosed with ADHD back in February. Which really explains some things, and why my grades in school were the way they were, and why it was so hard for me to focus. I was a shitty student. I never studied in school -- I never learned how. It never made sense to me. If I read something, it's like it flies in one ear and out the other. I have basically no retention. Staring at blocks of text gets me nowhere, it blends into one big blur.

I've been learning Japanese for years. It's been a dream of mine since I was a small weeb in middle school. I had textbooks, and I would read the lesson book, and then turn to the work book and again it was like I couldn't remember anything. I listen to podcasts while I'm working, those have been the best because I usually listen to them for a few hours, then go back and start from the beginning again, repeat, for days.

I have some Japanese manga I would like to work through. I'd like to study the kanji as I go. How are you supposed to study? Write notes? Do you read something, write it out, and then just... remember it?