I have 3 languages at C2 level (which doesn’t mean being native!) but I never studied them in the classic way so I’m not sure about the process. Not sure if I’m at C2 level in all 3 now in all areas of each language because I don’t use them so often but I feel very good with what I have.
Now I’m studying German at home and I struggle a lot but I’m asking a lot of questions here until I’ll find a solution to overcome my difficulties or I’ll give up and do something else.
For the others my process has been simple, sort of. Going directly to the country and learn them there. This doesn’t mean it’s been easy, not at all.
Usually I’ve done a few one-to-one lessons before going to the country just to have an idea of the language. Around 10 lessons more or less. A part from English that we generally study at school so I had a basic school level.
Once in the country, the process has been to find an apartment first, for example in France (or shared with other local people, in England). Then buying a bunch of grammar books and get out searching for any possible place where they could give free lessons. Create exchange lessons with local people that wanted to learn Italian and doing 1 hour each. Going to free places created for students that learn languages and so on.
At the beginning working a bit with my own language, for example in call centers and then working in hotels once I could have the possibility or a good level of the language. Depending on the moment, country, etc. I mean, I can’t explain everything but the method was this one.
To be honest, I did all of this without even being so determined and organized compared to today. Now I would do things a LOT better and I would use the time in a more intelligent way. I have wasted a lot of time when I was living in the countries instead of learning the language very well. And I did few “technical” mistakes, I wasn’t focused only in the language.
For example, when I was in France I was studying in Spanish language and when I was in England I had a French girlfriend living with me for a good part of the time. So, I could have done better. 
The bottom line should be that you need to know yourself very well. Then you need to know how you react with the language you’re learning. From there you do what you are doing, asking questions, trying to understand and then build your own method. That’s adult learning.