I live in a country where I don't speak the language that well (although everyone here speaks English) so I've been trying to improve it. Personally, it helps me if I have classes because it forces me to go and that way I always reserve some time to learn the language in a more structured way.
If that is not a possibility, there are resources online that you can take a look at. Apps like Duolingo, Memrise or HiNative, among others, can help you learn a bit of vocabulary and refresh your memory on things you may have forgotten.
Once you get to a point where you know the basics, then I would suggest reserving some of your time (even if it's just once a week) to work on your Japanese. Watching some series or movies with English (or Japanese) subtitles can help with getting used to the spoken language. Finding a Japanese penpal to whom you can write with in Japanese or English can also be of a lot of help, since it will force you to think and structure sentences in the other language. Some apps (like HiNative that I mentioned before) also allow you to record yourself and have a native person give you tips on how to improve your pronunciation. There are podcasts in Japanese (for example "News in slow Japanese") that can further improve your understanding of the language.
There isn't a clear answer, since each person is different and people learn in different ways, but these are just a few ideas that might help you! In the end is all about spending time with the language and getting to know native speakers.
Hopefully this helps you in some way!