I received wonderful advice from a friend of mine and I'm thinking of starting to make this my writing schedule also!
I would say to write often - start off once a week with 5 sentences and post it on italki for corrections. Every week or 2 weeks write a little more, a sentence, to get used to longer work at a manageable rate. While its good to choose random topics, my friend told me to use new vocabulary words every week with my writing. For example, if this week I'm focusing on learning french vocabulary for animals, I will write to include some of the words and new verbs in the weekly writing exercise.
Read the news! Like New York Times, the New Yorker, NPR (I'm biased because I'm from NYC haha) to see writing in action.
My friend liked writing stories in other languages to practice discription and dialogue.
I would be free to help out as a native english speaker!