Different people learn different ways, so there's not one answer. I think my main recommendations to consider over duolingo though are actual language textbook

Different people learn different ways, so there's not one answer. I think my main recommendations to consider over duolingo though are actual language textbooks, and then using any flashcard program for the information you need to memorize in the textbook (mainly vocab). I think another strong contender is taking classes, either locally or online, and getting to the point where you can consume media in the target language as soon as possible, even if it's children's books