You probably know this, but anyways:
- Lots of reading, both extensive and intensive. Find sources of comprehensible input: sentences that you understand 60-80%,
and will help you infer the rest by context. You will need lots of these. For instance twitter accounts in Korean, newspapers, etc.
- Watch shows with subs in your L1, or watch them 2 times: first time in L1, right after in L2.
- Use spaced repetition, but creating your on cards (active study vs passive study). Sentences better than words, bring context.
- Do some writing. Perhaps a journal, or something about your job / field of expertise. Notice the gaps (things you want to say but don't know how). Get it reviewed by a native.
- Have regular 1:1 sessions with a tutor / teacher, eg. on Italki.
If it is important to you, it is worth to spend a some money on it IMO.
Good luck!