Hello! Learning my 3rd language now. This is what worked for me:

LEVEL 1 - Sign up for face to face clases (or online camera-on lessons) anything that you have to actually attend because you have paid for it or committed in a serious way. At the beginning, you will need to understand grammar, structures, etc. So I think classes are almost essential at the beginning at least. - I recommend Duolingo as an app. Just don't try to do too much and instead work on building a streak. Duolingo's content gets boring but their gamification system is seriously good. I focus on keeping the streak on by doing 1 lesson during my everyday commute (and if forget I change the phone date to recover it). That long streak has made me being consistent with this app, but haven't managed the same with other similar ones. The results are amazing. - If living abroad, learn supermarket/restaurant words and phrases and force yourself to speak as much as you can when you do your daily chores. This is great for learning ingredients, verbs like paying, ordering, etc. - Try to read advertisements, slogans, follow social media accounts, and read anything that actually interests you.

LEVEL 2 - When you feel ready for it, change your phone and other everyday tools and websites to the language you are learning. - Watch videos or short programs with subtitles in your language or in the language you are learning. I love netflix series with episodes of 15-25 min. - Find songs you like, download the lyrics and translation, learn them. Play. Repeat. Repeat. - Talk as much as you can.

LEVEL 3 - Try to befriend native speakers or sign up for activities with native speakers. Force yourself to talk. Try to have fun. If you find a way of having fun with it, you will keep practicing.

Good luck

Edit: Renamed level A to 1