Read more. The number one way to improve is to absorb vocabulary and phrases from reading.

lots and lots of practice on lots and lots of different topics

In my opinion, improving your writing ability has multiple parts. - read the type of writing you want to write (academic papers, novels, news articles, etc) so you can learn style and expressions. - write often, even if it's only a short amount. - get feedback from others about your writing (a teacher or peers) - read your own writing from previous weeks or months to see how you're improving and what areas you still need to improve.

Have you considered taking a course with an experienced, qualified and engaging tutor? You may not be aware that italki has the word talk in the name, but half the lessons I teach are actually based on reading and writing - as well as speaking and listening. I love writing. I would be delighted to see what you need to do to get you to where you want to go. I've been doing this for 30 years all around the world, also with Chinese students in China - and now online for 14 years. I sometimes teach multiple generations from the same families and many of my students engage me for years. Most of my students come by recomendation. I tutor adults, young adults and children as young as 4. My clients even become my friends. I make every course myself according to the needs of each student using books, videos, interactive, practical follow-up through WeChat, etc. I use Tencent and zoom for better meeting quality. I have a website in China too. Hit me up for a trial class and we'll build you a plan of action. In that piece I used slang, idioms, phrasal verbs, expressions and correct punctuation to create the intended depth of meaning. See you when you are ready.😊

If you continue Practice and learning. Definitely improve your writing skills.