Consider obtaining a Bachelor's degree in creative writing from an accredited university. Although this is not a requirement for becoming a mystery writer, obtaining a degree will help you hone your skills as a writer, create strategies for writing and inform you about the industry that you are trying to break into.
Read deeply in the mystery genre and understand current trends in mystery writing. By reading some of the historical novels that were extremely successful and understanding the current developments in the mystery genre, future writers gain the skills needed to construct a well-crafted and believable mystery.
Develop a firm grasp of the mechanics of writing and the standards of the genre. Mystery novels need to be well written and conform to the tenets of the genre.
Build an understanding of the differences between the sub-genres of mystery. For example, a locked-door mystery is very different from a cozy. A locked-door mystery is a type of story where the crime that is committed is done under apparently impossible circumstances, while a cozy is a type of mystery novel in which the sex and violence of the plot are downplayed.
Research the topic that you are going to be writing about. For example, if your mystery novel is going to take place in the city of Chicago, you should make sure that you know all about the city. The only way that readers will believe your mystery is if you present realistic images of all the things you are describing.
Develop an interesting, exciting and suspenseful plot. Many mystery novels feature a detective as the main character, but this is not necessarily a requirement. Create a plot that is interesting to you and devote yourself to building it and making it great.
Plant clues within the plot you develop that will allow your readers to try to figure out the mystery if they want to, without giving away the ending. This will help make your mystery novel exciting and engaging.
Write your novel. This part of the process can take a lot of time and will require a lot of effort, but for most writers this is the most exciting aspect of the job. Write a first draft and then review and change the draft as much as you need to.
Search for an agent or publishing company that specializes in the mystery novel genre and work hard to get your book published.