Your past experiences may help me improve. Sometimes I spend a lot of time looking for one word until I give up.
Do you keep Sheets with important words? And how do you learn new words?
Was your writing style bad at first, or were you naturally good?
I wasnāt born with english. I speak 4 languages. Uranus birth tongue wich cannot be written. Then the other language from the Invader lol which was crap and I did everything to forget it. (You canāt alas lol). Then french, and finally english.
When I started learning English it was in school. Then stuff happen and my education stopped.
When I finally picked english again, it wasnāt back in school here where I live (Canada), the teacher kicked me out of her class. And I dropped out shortly after for reasons.
I remember a friend sold me a copy of Baldurās Gate 2. And it was in English. That was my inspiration to get back and learning the language since when I tried playing it, I didnāt understand squat.
I also used (back then it was crappy, today translators are better) Babel Fish Translator. I used it to translate a fanfic I was reading, word by word. Then started watching movies and documentary in English. And never stopped since then. I even wrote fanfics about Tv show even if my English was crap.
But if you keep at it, you improve. Especially nowadays. You have the luxury of the internet. There are tools that are free like Grammarly which you can install and you can learn from it how your spell is going.
You can check videos on YT and learn more there.
But there is no fast trick, you just keep at it until your own dreams are in English with whatever your birth language is as subtitles.
Link: https://app.grammarly.com/
Link: Learn English Online: Free Courses for All Levels - Busuu
No. I donāt use sheets. ever.
By reading books, magazine, talking with peoples, newspapers, from movies, Tv showā¦
I mean just think about how peoples learn words just from reading mangas.
Oh it was. Horrible even.
I was never good at anything, thatās why I dropped out from school. I sucked at everything 
Google is your friend. If you are looking for a word like say āāEatāā. But you canāt remember it ? How about you look it up in your own language? in french it is āMangerā.
If you have the internet to post in this forum, then there is a gazillions of resources out there for you to dive in and learn and absorb.
Anyway, this is my experience and I do wish you all the best! 
Beta testers and actually applying their suggestion to my writing. Donāt you ever look up my first game because my goodness that thing is bad.
One of my weaknesses among others is adding āSā in verbs that donāt need it and not adding it to the ones that need it.
I remember one timeā¦ the whole comment from the tester was:
āSāā¦ you forgot āSāā¦ take out this āSāā¦ āSā Oh my god! āSā!!!.
I feel like my grammar has improved since then, it still needs work, but now Iām able to recognize some of the errors myself and provide less typo-riddle work to my testers. 
Soā¦
1: ask for beta testers, and tell them exactly what kind of feedback you need from them.
2: use Grammarly or any other spelling and grammar check app.
3: If youāre unsure of something, google it and look it up.
4: Be aware that some words in your own language donāt translate the same way in English as inā¦ they donāt have the same meaning.
5:Make sure to stick with one style. The UK English style or the American English style because they have different spelling for words. (Color-colour, pants-trousers) Make sure to know because you might get testers from both styles, and while they are both correct, itās not really fun to mix the two styles in your story. (Maybe thatās just one of my pet peeves.)
6: Post your WIP, people in this forum can be very helpful and are often ready to provide screenshots with all the typos and corrections (Thatās my favorite part because it helps me make the game more polished.)
Iāve been fortunate enough to have some close people that I can now call friends to help me with my grammar, and Iām thankful for them.
Donāt get discouraged, some great novels are published with typos .
Thank you all. Your experience motivates me a lot to continue working on myself. Even though Iām 21 years old, sometimes I feel that Iām too old and that I should have started way sooner when I was much younger.
Pretty much switching my life to English, or most of what it could be changed. Watching YouTube, series, movies, whatever in English with English subtitles, reading and writing, thinking, research, notes. Only stuff like work in which no one uses English and speaking with local people, all the other stuff, switch to English.
As for past experience, I had English at school, but the last 3 years I switched school and I started to see everything that I had already seen on the other one, so not much progress there.
It happened to me that I liked the TV show Mythbusters, and I found out that the episodes that were airing on the local TV channel was 3 seasons behind the original cause of the doubling voice-over they do to switch languages. So I got the new episodes but I couldnāt find subtitles at that time, so I got kind of angry and started to watch it anyways. To my surprise I understood like close to 100% of everything. Then it happened also with Malcolm in the Middle, I got the series but no subtitles, so those two were a pivot point for me. After that I always tried to look for English subtitles or no subtitles at all.
And starting to write my project for HG helped a lot to push me further too, the more you get your hands on the language you want to learn the better.
Hereās something that I found really interesting that may help you more than me telling you my experiences, itās more based on science and people that knows more than me about what theyāre talking, so.
I wasnāt a native English speakerāit was my third one, I thinkābut I did learn it fairly quick. But learning how to write creatively vs learning how to read are very different things, as I discovered the hard way. Iād say you have to consistently read novels/short stories (or watch stuff) that have been praised for their prose and the development of their characters. Itās a common advice, but they really help a lot, and by viewing them as a writer, I honestly believe you can learn things you wouldnāt learn at school.
I use this website to find words that I need but canāt seem to remember. Basically, you put in a short description and it can provide you what you want! Also, this is mighty embarrassing, but after my first try at writing, I basically picked up the book I loved the most back then and copied (by hand) every single paragraph that had great setting descriptions, character dialogues, and action sequences lmao. I studied them every night like Iād study for an exam, though I think it did help? kinda? Itās not something Iāll do ever again because wow that was absurd.
I was insanely bad. My first attempt at writing was when I wrote a screenplay at 13 for a local film competition, and well, sometimes I find that script every now and then and I justā¦ cringe. It was probably not an appropriate age to join something like that, but it was definitely aā¦ learning experience. (I just hope anyone who ever saw that film has already forgotten about it.)
I donāt think there are shortcuts to learn more about English, but it will surely help you to keep writing anyway, even if youāre not feeling confident. Youāll get better as you go on!
Great advice. 
And thank you all for videos.
And thank you for the link; this website should help me a lot to look for new words and the ones that I forget.
Not a native either and the practice that helped me greatly was the same as most of the others on this thread: practice, practice and practice.
Besides doing the obvious of reading a lot of novels, what helped me tremendously in terms of both vocabulary and how to write in a more prosaic fashion was consuming a lot of essays.
Essays, when well written, are often fantastic ways to expand your knowledge base of a language, since their primary goal is to convey not only an opinion but often the emotional impact attached to it. As well as trying to keep things engaging to the reader. Sounds a whole lot like reading a good book to me.
I, for one, greatly enjoy video-game, literary, art, philosophy and cinematography essays on Youtube. Channels such as: Leadhead, Cinema Cartography, The School of Life, Jacob Geller. To name a few.
I tend to stick with longer videos (unless if the topic doesnāt have much to talk about) since it puts the onus on the creator to string all of their narrative in a cohesive fashion, which is in and of itself an exercise in writing.
Hope this helps. 
Does an English-English dictionary count?
Reading and googling words or proverbs Iām not familiar with.
It was, and still is, bad, but itās getting less bad? I have a number of writing-related points for improvement (for example alternating between dialogue and description, how to include body language, pacing, worldbuilding, etcetera) that I dedicate practice time to for example through writing prompts and analyzing the writing of other people who do it better.
Read. Read. Read more. Read varied. Read old books. Read new books. Read books by men. Read books by women. Read books from other cultures. Read books from different genres. Read literary fiction. Read some classics. Read some books that are out there. Read some poetry. If you donāt understand a word, look it up, then move on and read some more.
Also, figure out why you like what you read. Why you donāt like it. Not just the characters or the stories, but the words. Write down cool quotes. Take screenshots or pictures of your favorite pages or passages. Collect cool beginnings and first sentences. Figure out what makes a satisfying end.
And then: Try to write like your favorites. What makes a passage from their book sounds like theirs and not anybody elseās? How long are their sentences? How varied? What kind of tricks do they use? Write about one of their characters, but from a different viewpoint, or a scene you didnāt get to read. Figure out how they make their characters talk.
And finally. NOBODY is good at the start. We all suck. But some people you get to see later in their writing life than others.
This stood out to me very prominently when reading Bram Stokerās Dracula. While I liked the overall style of his writing, it was exceedingly repetitive. He couldāve easily cut out several paragraphs out of the novel with some mild editing.
Another author that had a rather quirky style at the beginning of his career but developed a more naturally flowing prose is Lee Child of the Jack Reacher franchise. His first works had very truncated sentences all punctuated with periods. It did lend to giving the main character a clinical and robotic mindset (which does fit narratively) but good lord was it a bother to read.
On the flipside some of the authors Iāve adored reading for their cohesive and straight-forward albeit striking narrative were Charles Bukowski, Brandon Sanderson, Neil Gaiman and Terry Pratchett to name a few. If I want a highly poetic and abstract form of writing then Clarice Lispector is my absolute go-to with Nietzsche a close second.
For me, most of it boils down to the fact that Iām a huge bookworm and I read a lot of books in English. Thatās how I expanded my English vocabulary, by guessing what new words mean by context clues and then checking a dictionary. Iām not sure why, but I tend to recall new words easier that way when compared to just straight-up translating a word. Itās also how I improved my writing style.
Honestly, I think the best way to expand your vocab is to find a piece of media in your target language (in this case, English) and figure the meanings and such from there. It makes for a more fun experience, and personally, those stuck more than what Iāve learned from school. I do occasionally have to go back and remind myself what certain words mean, but for the most part I can remember them naturally.
My writing style was shit (still is lol), but Iād like to say that Iāve gotten better over the past few years with practice. I no longer use the same words an excessive amount of times in the same page and while I still struggle with description, Iām definitely better than when I first started writing in English.
Honestly Iām just naturally verbose
I go back and read stuff Iāve written years ago and itās like a reading assignment in law school.
Same way you improve with any language. Hours and hours every single day of extensive practice with reading, writing, listening, and speaking in a variety of contexts.
I would like to add one more thing to all of the other excellent answers above: be aware of how your native language influences you in constructing sentences. When you think about a sentence, your brain may default to the way you would describe things in your native language and when you write it down you may not notice it and it may sound strange in English. To correct this bias, make sure to pause for a few seconds after you write something and then read it. If it sounds weird, then think about why it is and how you can change it. In time, you will do this automatically and effortlessly, but it takes practice, just like learning any new skill.
Iāve always had a decent vocabulary, but i think thatās because I read books that were always for above my age; so i began to expand it rather early. But, to be fair the more you read the more your vocabulary will expand. See a word you dont know? Look it up.
And when reading, do it with multiple genres, since each one seems to use different adjectives and adverbs more often.