Imagine you're reading a book: they're not all the same. There are good features of writing to describe which can help you, and check-lists you can follow, but ultimately your piece of descriptive writing will be unique to you. That's what is so good about it; you can be as creative as you want.
A good place to start is to think about how you open your piece. What is it about? You might want to open with a snappy introductory sentence, for example, which would allow you to write more as you continue writing.
e.g. It was dark. The clouds around the turrets were the only source of light in the sky, and I wondered how they looked so peaceful.
Sure, 'It was dark.' doesn't seem like a very exciting sentence, but followed by the development it sounds intense, spooky and fits the theme of spooky descriptive writing.
Next, you should think about inserting some literary devices, for example similies, metaphors, alliteration, list of three. Literary devices simply allow you to make your writing sound more sophisticated and interesting and can be really easy to include in your writing once you think about them.
Using these techniques will help you write a brilliant piece of descriptive writing to win you big marks in English!