How can I improve my English writing skills?

I think the only things you need is practice and exposure to English as much as you can.
Many people believed that if you read a lot, you write better. That is. Just read until your eye burnt out. When I ever I try to search something on Google, I searched for it in English from fanfic, movies reviews, books, cooking recipe, exercise… I immerse myself in that language. I write what I read and hear. That why movies have been my mentor since day one too, and greatly influenced my writing and speaking. But I can’t tell you to watch movies only because I read a lot. My reading and writing skill did become better.  Go on Google and search for all the well-written book out there. Start with your favorite genre but later try on different types too.

But to be honest, it didn’t affect much of my writing. I can only write with simple words that 6 or 7 grade would be able to understand.  I still working on that part.
The best way is to practice every day too. If you read about like 50 pages, you will be able to write a page. That is my case. You need to have an input. The flow of information. You need to read and listen a lot to the topic you want to write about. So you can be able to express precisely and beautiful. Consistent is the key to learn anything. Try to practice every day. Write a sentence, a paragraph or a page. It’s up to you!
Don’t force yourself too. These things take time. You won’t get better in a day or two. Everybody has their own pace of grasping things. In time, you’ll get the hang of it because you will have developed wide-ranging and different thoughts. I still have to improve a lot too, but if you love the language yourself, you won’t feel like learning at all.
Have a diary, or a journey would be great. Keep your old writing then one day when you read back you will see how much you have grown.