There are some reason prove that writing can improve your English skill significantly:
1. Research show that you can remember more efficient when you write down to a paper. There's a connection between your hand and your brain.
2. When you write, you know the way to select words, put words together, you know how to order words, and it sculptures to your brain. And then when you speak, your brain already knows the way to put words together, it becomes automatically and then the sentence come out of your mouth naturally.
Some advices:
1. Don't try to be perfect. If you want something to be perfect right at the beginning time, you will end-up frustrated and more likely to give up. Accept mistakes, keep going and gradually you can build sentences correctly and quickly.
2. Just write as you speak. No need to write complicatedly, just simple sentences like who? when? what? how?... and then by the time, you will be able to write sentences which is more complicated.
3. Build your habit of writing in English any time you can
