If you think about it, and know what to do, creative writing isnt so hard. Basicly, creativ writing is writing in any type of genre, in any form, but outside the box, and your own thinking on it.
For example, if your teacher assigned you to do a short story of a more modern folktale, you might first think of what old folktale you could modernize, or improvise it. Say you chose Jack and the beanstalk. Think of things in the story to change, and make them seem more like its the time we are living in. For example, instead of jack stealing a goose that lays golden eggs, he could have stolen an iPhone that has extra space or something that would e more common but still something someone would really want.
Next, after you have planned things out, you can start writing, when you write, make sure everything you write connects well. Like, on a new paragraph, make sure it somewhat connects to the previous one, so that it makes sence and the reader is still in contact with what they are reading. Also, make sure they know the characters and their rolls, and make sure to tell them early in the story. Also, have a huge beginning, and that will effect how the people continue reading. If its something eye catching, then they will want to continue reading to find out what happens. And make sure you are writing from yourself. What I mean by that, is put your own twist to it, and make people know that you wrote it. Make it special.
In a grammar form, make you that you are spelling correctly, so I would reccomend going over everything when you are done, and if you are writing online, you might use spell check just to be sure. Another thing that might help, is if you have a trusted friend or family member read it and give you tips or corrections to make it even better. Also, make sure you are separating things into paragraphs of at least 5 sentences. When you do that it makes reading a lot easier in a format way. When you are done with your whole story, I reccomend reading over it yourself and thinking, "What can I do to make it better" or "Is there anything I can do to make it better" If you feel confident with what you are doing, then, you are done!
After my tutorial, I hope that you have a better view on how to do creative writing. Just use your own creativity.