[jamager]

Very very summarized (the devil is in the details):

From 0: phrase books + spaced repetition + easy recordings for beginners (many webs provide them for lots of languages). Don't forget to work on listening and pronunciation

A1 - A2: add active reading with graded readers + some writing

B1 – B2: keep on graded readers and SRS, add Italki tutor, podcasts and new learning materials. Mix passive and active reading / listening. Start looking at grammar

+B2: lots of comprehensive input + whatever you fancy (eg language exchanges, creative writing, etc)

If you want to know more I wrote a book here: https://thehardway.guide/ (with lots of links to other books / sources)