People learn English best when they're forced to do it because of their surroundings. Fluency is like physical fitness in your mind and also in your body. You can observe the way other people write their emails and kind of copy their style. Write with a reader in mind, even if nobody reads it. This will help you get used to simply putting your ideas into words. Building sentences is a sort of art or craft. You have to practice it in order to get some level of comfort with it. Keep that part of your brain fresh by writing English as much as you can. And that's as scientific as I can get here.