Ask for reviews as often as you can.
You can’t know how well you write and how much you have improved if others don’t read your words. If you don’t want to share your compositions with your friends, you can publish them on special websites under a random name or you can share them on social media. Ask the readers to give you honest feedback and to tell you what’s missing from your texts.
You might have great ideas, but this does not mean that you can afford to make too many grammar mistakes. Your teacher will pay attention to this and you will never have great marks unless you make an effort to learn all the things you missed in your grammar class. You can take an online course and get back to the basics, or you can ask your professor or one of your colleagues to explain to you the things you don’t understand.
Books, magazines, stories? Anything works! Without realizing you will understand how you have to build a sentence in order to look right, and what are the types of texts that most of the readers usually dislike. You can choose the books that you love the most, and in this way you will spend your time in pleasant way and you will improve your writing skills at the same time.