As Tess posted,
go to France and speak french with french people
I think is a very good way to do it.
I took a job there, lived and worked their for 18 months.
When I first arrived, I spoke no French at all.
I didn’t generate as much of a command of the language as I’d have liked, and some people at work would speak English.
However, outside of work, I was left to look out for myself and mostly I enjoyed it.
I always had my DK phrase book with me (pocket sized and very useful), for when I got stuck and once people saw me reach for a book and try again, rather than expect them to speak English, they were most patient and helpful.
Spending time there, living there, certainly worked for me.
Same for other countries I’ve lived and worked in really.
Being there was the best way for me.
🙂