I’ve been learning languages compulsively for thirty years, starting with Czech and Russian. I can’t say I was successful. I would write down new words in a notebook or write notes in margins. After a while, I started making flashcards using index cards. If I got the answer right, I’d move it to the back of the deck. If I didn’t, I’d move it closer to the front. It was my own spaced repetition algorithm. But that’s a lot of index cards. ReadLang is much better!