Native English speaker - Got invited to a wedding Poland once, met a girl, got married, had a kid and now I speak Polish at an upper B2 level.
I started learning when our daughter was born. I got a massive boost because until my daughter went to school, my wife only spoke Polish with her and I only spoke English with her. So I was constantly hearing very simple words/phrases very frequently. My daughter was also watching kids shows in Polish and we had a lot of Polish children's books in the house. I did try things like Duolingo or Rosetta Stone but the stuff that actually stuck was the things I picked up in real life. I talked to myself a lot as well. Whenever I was doing something I would ask myself "Can I describe what I'm doing right now in Polish?" If I couldn't then when I got home I learned how to say it. By doing this you learn which words are most useful specifically for you to learn first. I focused more on taking in as much input as I could instead of studying grammar rules from a book. I still can't really tell you the grammar rules in Polish, at this point I just know it sounds right. I subscribed to a few Polish channels on my cable service and listened intently while my wife watched her shows. I listened to music in the car. I listened to learning material while I went for walks. Basically I created a little Polish world in my house.
After a number of years I finally started working with an online tutor to help get to the next level. I'm glad to say that I've never had a conversation with him in English. We travel to Poland every year to visit my wife's family. For the past 4-5 years, I switched and now I only speak Polish while I am there. At first my brain hurt and I was exhausted but like working a muscle on your body, I got use to it and now it is fun.