You write a myth in much the same way as you write any story. This means the characters and the setting have to be as important to you, the writer, when you write a myth. Most important, edit and review the myth to make it perfect.
A myth is a story that has no basis in fact, but is presented as such. Myths can be explanation stories and culture-related stories that help give a society a sense of identity. They are also used to explain a group’s origins. They are also stories that are commonly accepted to be true, much in the same way as urban legends.
First, you need to address the function of the myth. Ask yourself, what is your reason for wanting to write a myth or urban legend? There are two main reasons for doing so. The first is to form a background story for a piece of fiction, such as the mythologies of Middle Earth in ‘Lord of the Rings.’ The second is so you can fool all manner of people, across the world, into believing something that is patently not true.
Second, understand how myths and urban legends work. The story needs to have an appeal to the readers, it needs to stir their emotions in some way and it needs to be plausible. One of the major elements to be kept in mind when you write a myth is that they are cautionary tales. Ask yourself, what do the people learn from the myth?
Write down the objective of the myth and the key elements you have in mind. Write a brief summary of the story and examine its plausibility. Work out the setting. Do you want it to be generic like a restaurant or a hotel, or part of a specific cosmology, such as based in Asgard or Purgatory?
Develop the characters involved in the myth. A good myth, especially one meant to fool people, has a small cast of characters. This is usually around two to three. They are also based on strong archetypes that are understood the world over. Think of the salesman on a conference, the attractive woman at the bar and a ruthless gang and you get the myth of the man whose kidneys were removed.
Keep the urban legend-style myth short. When you write a myth like this, it is important for it to be snappy and generic. People will then adapt it to their own locality, so keep the important details, but do not be too specific.
When you write a myth as part of a mythology, the story can be longer and more complicated. Keep it consistent to the culture setting. Remember that some elements can be taken as understood by the audience, but others will need explaining.
Check the story. Remember that factual mistakes undermine it and make it less believable. Get friends to check it too then present it to the world.