To write a fairy tale you need to know your fairy tales. There are several out there, many that are rarely heard of. There's French, German, Chinese, and so many other tales from around the world. Even the Brothers Grimm and Hans Christian Anderson have tales that no one talks about today, choosing instead to focus on Snow White and Little Mermaid. Start by reading all the fairy tales out there, and each retelling, starting with authors like The Twisted Tales series by Liz Braswell, The Woodcutter Sisters novels by Alethea Kontis, and Alex Flinn's Beastly.

Be original. Put a fresh spin on the fairy tale, take only a few elements and find a way to spin them in a whole new direction. Stay away from the popular stories of Cinderella, Snow White, Red Riding Hood, Beauty and the Beast unless you have a really good idea that hasn't been done before. Cinderella is one of the fairy tales that everyone knows and that has been retold countless times. Before writing your fairy tale, determine what you want to say and how to make it different.

You can set your fairy tale in a modern time, or play with another decade. Maybe put Hansel and Gretel in the 1950s or Rumplestiltskin in the Wild West. A lot of fairy tales are set in high or urban fantasy, like how Alex Flinn spun a modern day Beauty and the Beast with a blend of reality and magic. Use a completely different setting, retelling a tale from another place in the world, or create a fantasy world that has curses and magic as the focus, or go for a more bizarre futuristic dystopian.

Try out a villain's perspective. Maleficent retold the story of Sleeping Beauty entirely from the villain's story, putting the well known witch that cursed Aurora as the one that was wronged. It turns the villain we love to hate into sympathetic hero. As you read a fairy tale, ask yourself how could it be different. What would it be like reading Hansel and Gretel from the Blind Witch's point of view? What if Cinderella's stepmother won? 

Pick a side character that barely has a role in the original tale. There are several versions of Cinderella that highlight her wicked step sisters, but what about the mice she seeks for help? How would Snow White be different if the seven dwarfs told the story? What does Cinderella's godmother do when she isn't helping Cinderella attend balls? 

Try a prequel to a tale we already know. Tell the story of the witch that cursed the Beast before she met him, or tell a story completely about Cinderella's fairy godmother. There's also the option of mixing together different fairy tales into one world in the same way that Once Upon a Time does. Put a fresh spin on all the stories we know, and don't be afraid to not have a happily ever after. The actual fairy tales are darker, and don't always end merrily.