simple, read stories that challenge you
read stories with as many words you don't understand as possible
and whenever you don't know a word (which will occur pretty often at first) just use google translate (10/10 would recommend that app 😤👏💯)
if you want to improve your english by reading good taekook fanfics just go on archieveofourown.org and type in playfultae
scroll down on my profile and read the ones I gave kudos to
I've noticed that ever since I started reading on ao3, my stories have been much better vocabulary and grammatically wise
so yeah, read a lot and pay attention to how other writers write about specific situations
and if your native language isn't english: don't worry, you'll learn too!
I've learned english through wattpad (yes, I'm garbage) so don't give up!