Being able to think in your target language—for example, Korean—is one of the most important steps to making your speech more fluid and natural. The only way to get to this step is to practice, honestly. The more you practice, the faster your brain gets at piecing together and producing the sentences.
Think about back when you were little, when you first learned to write (or even when you first learned to write Hangul!). At first, you had to think about each line, and it was slow and awkward. But you kept using it, and now writing is just an automatic process, your hand moving effortlessly to make the write forms. This is a really simplified comparison, but you see what I’m getting at, I’m sure!
It’s easy to say “just practice more!” but I know that not everyone goes to classes where they have time to practice speaking with a teacher, and not everyone has native speaking friends to chat with. So, you have a few options. If you do have native speakers around, great! Talk with them a lot. Just talk and talk. If you find that you don’t know a word you want to use, instead of just saying it in English (or whatever other language), try to explain your way around it in your target language. Also, you could maybe try talking to yourself. Maybe you’re at home and you’re getting ready for bed. Take some time before you go to sleep to talk to yourself about what you did during the day. Try to go through everything as best as you can. Even if your grammar comes out wrong, that’s okay! The goal is to get yourself speaking for longer periods of time without your brain reverting to your native language.
I hope this helps you out! Happy studying~