By doing. Try to write, read, speak and listen to large volumes of English. I've written blog posts, chatted a lot with people in games (mainly text, sometimes voice), talked English at every opportunity (e.g. I'd seek out non-Dutch people in class and do projects with them), wrote elaborate answers on the security stackexchange website, listened to audio books, etc.

I didn't have good experiences with apps that try to gamify the process like Duolingo, but that might have been because I used it in semi-corner cases: in English I wanted to go from mediocre to advanced, and for German I already knew Dutch so most of it was super basic and didn't really help me.

I've also looked for other hacks to get better at a language, but the thing that really helps is just by doing. I guess audio books are sort of a hack, because you don't have to be very actively busy with it: you can put an audio book on while doing laundry, dishes, driving/walking/biking, falling asleep, etc. And it teaches you vocabulary (most unfamiliar words are clear from context), correct word use (including proverbs/sayings), and pronunciation. Unfortunately, I noticed later that I don't know how to spell those words, but that's just a minor issue (easy to look up when needed). I still encounter words randomly and think to myself "oh that's how it's written! I just read this sentence fluently but I never read this word before".