spoiler alert: there is no magic bullet. it takes many years and is a lot of hard work, but it's worth it!
i see a lot of posts of people not sure where to start. this is one reason why duolingo is so popular. it does the hard pedagogical work that you've probably learned to take for granted from school/teachers. guiding your attention and giving you the next step that's just challenging enough to be interesting, but not so challenging that you give up.
however, duolingo is, in my opinion, a dead end app. it will trap you for years in tasks that are too easy, won't explain key concepts you need to know, and has all but dismantled the spaced repetition features that used to aid memory. if you take nothing else away from this, let it be that duolingo has all of the disadvantages of school language learning (slow progress) with none of the advantages (good challenges, thorough guidance).
so what should you do instead? it can feel overwhelming, but here's a rough guide without being specific about links or languages:
- find your why. there's a reason language teachers joke that the best way to learn a language is to fall in love. you need a reason to be learning the language that will sustain you through the periods where it is not exciting. write down what you ultimately want to do in that language.
- find music or a tv show with subtitles. start getting some audio input. it's okay to not even try to understand. this is to train your brain to the statistical probabilities of where words end and begin. do some every day even if you do nothing else.
- if you have time beyond your daily input goal, it's time to work through a real textbook or high quality online resource. buy a textbook or get one from the library. find online lessons and feel free to skip around, keep lots of bookmarks. it doesn't really matter that you do things in a particular order as long as you learn something new every day. using lots of different types of resources is great. try some workbooks or online resources that have drills. take notes, make anki flashcards for vocabulary and get audio from forvo.com.
- after a few months of steps 2-3, add graded readers to your routine. if you reach something that's too hard, take a break and do steps 2-3 only for a while, then try again. when you feel like the highest level of graded readers are too easy, start buying or borrowing real books at an elementary or junior high level. search for your target language and "books for (five year olds)." take notes when you read.
- start mixing in podcasts and try out watching tv and movies with target language subtitles.
do a little SOMETHING every day. it really doesn't matter what. it will take a very long time, but language learning also has a stair proficiency effect, where suddenly you will feel like you've reached another plane of understanding all at once, then a long plateau. track the time you study, it's less frustrating than having "finish xyz" goals that might not get finished. with language, every little bit truly helps. consistency!!!