Kirirao,
Improve your writing skills? Write a book. I am serious. When my Japanese language ability was "much lousier than it is now" I began writing a book in Japanese. What a mess! I had someone proof-read each sentence, and it was a painful process.
In the beginning I had to compose each sentence in English in my head, then try to translate it into Japanese. By the end, I was "kind of" composing the sentences in Japanese in my head.
All it takes is practice. Write something long and get it proof-read.