In my experience, a textbook and flashcards are great for your first 500 words or so and just enough grammar to get your bearings. I also do a lot of listening

In my experience, a textbook and flashcards are great for your first 500 words or so and just enough grammar to get your bearings. I also do a lot of listening and phonics work at the beginning. After the first month or so, I find extensive reading (with graded readers) to be by far the fastest way to pick up and cement vocabulary gains. Listening (to radio or podcasts) is harder until you building a larger vocabulary, so I gradually add it to my reading habit as I improve.