First of all I should say there isn’t a silver bullet to solve all problems one must face to learn beyond the basic English. Save yourself a boring reading if you are looking for that on this post.
To master and learn a language is no easy task and I admit both my Portuguese and English could be way better if my education was higher and the environment where I was born nurtured it from the very beginning.

When I was younger I hated school with all my heart, so much that after the basic required by my country I went to work for my father. My first job required a “hands on” skill learning and no need for language skills.
Sure that I was well paid for doing my job after I mastered it, but I should have continued school and was dumb for not doing so, because my English and Portuguese could be way better right now.
So without much options and with a new full-time job, I had the need to at least improve my English because it was a important skill at my new work, thus I started to learn English and I continue to this day.

I’m not the best example of a student. I’m lazy as hell, my grammar sucks, the structure of my writing is wrong most of the time, even so I keep going because one day I’ll be a better writer.
There is a Russian artist by the nickname Dzikawa that often says on her Twitch streams “Just Do it!”, besides referring to a fun viral video of Shia LaBeouf screaming “Just do it” at full lung power, she is also saying that people should try and try again until their practice becomes second nature. I agree with this because even an old dog like me can learn a trick or two with enough practice.

I already wrote on my blog how I buy things on eBay, in fact I also use it to find good English books to use as reference when I’m writing for this blog.
For example anything by Oxford is good, The Times style and usage guide book is a must have even if a bit impractical.
Just avoid any legalese related books unless you are a lawyer or want to be one!

If possible read in English whenever you can. My writing skills developed after reading a lot of English novels and biographies.
You’ll start to be aware of little details and even correcting some things that seem a bit off or strange when you read them out loud.
Always read stuff you truly enjoy, because that way you’ll have a reason to push your studies forward.
Don’t like reading books or magazines?
Read websites, blogs and go to chat rooms where English language natives hang around.

Disable subtitles on movies and series and above all don’t hear dubbed stuff, to learn English one needs to know the spoken language and translate it in real time, the best way to do it besides movies/series is to practice with real people, in that regard I’m lucky because I got a job which allows me to practice all the time.
Most cities have workshops and groups of people who gather to help and learn new languages, so it might be a good starting point if one isn’t shy.
Having friends abroad is also a good way to improve both spoken and written English, you will be forced to reach out and understand them.

Last but also important, if you are European you will probably learn British English at school but be exposed to American movies and series. Nothing can be done to avoid this so speedway/motorway, color and colour between many other options will have to be dealt sooner or later if you want to write in English. Choose a country and stick with it.

On a last thought about this subject, English teachers are the best ones to correct and point you on the right path, cherish them.
Learning alone is boring so find someone to write to and practice with if possible.