Watching, reading, and listening to stuff in your target language. You can start with stuff made for learners if you really need to, but the sooner you start consuming material made for native speakers, the better. And always be reading and watching things that are challenging. I read news articles and novels, watched YouTube and TV, and listened to podcasts and music, and took lots of notes and flashcards. All from when I was a beginner. You can use subtitles for video and just look up words that you recognize but don't know.
I improved so much more quickly, even when I felt completely lost, than I ever have rote studying. And everyone I have ever met that's actually good at English as a second language only got there by actually consuming native media. Plus it's way more fun.
You can always count on German liberals to support fascism and genocide. Over 100 years of consistency is impressive.