Come up with an idea. This is, of course, the most critical part in writing your murder novel. You need to decide who or what you are writing about, what events are going to occur, and how the book is going to end. While you eventually need to know all of this information, it might not be something that makes sense right away. Just start writing and allow your work to take shape. You may be surprised at what you might find.
Decide how you are going to tell the story. Often this step will sort itself out as you start writing. But it may be helpful to decide whether you want to tell the story in first or third person from the get-go. This may save you some time in the long run. However, there is no fault in not being quite sure until you start the process. Most novels follow a linear path---they start in the beginning, the conflict is introduced, the climax addresses the conflict, and then a resolution is offered. However, you can start in the middle or the end and make sense of it all later. Such a juxtaposition of traditional plot structure can actually work quite well in a murder novel.
Come up with your characters. Every story has a protagonist and an antagonist to create the conflict in the book's plot. In many murder novels, the protagonist is the character trying to get away from the killer while the killer is the antagonist. However, don't let that limit you. It is possible to write a page-turning murder novel from the perspective of the killer. It might be helpful to draw character maps to help make sure your characters will act in a way that is consistent with how they are being written. For the most part, your characters will seem to take on a life of their own. Don't stop them---allow the writing process to really guide you on your endeavor.
Name your characters. For some, the name of a character is a jumping-off point. In fact, many creative writing exercises begin with the writer penning a story about a character with nothing but a name as inspiration. However, the name of a character is not something to get held back on. Many authors don't name the characters until the epilogue is written or the sequel is in the works. There is no rule for this process other than to be creative in fitting a name to your character.
Build suspense. A good murder novel always has suspense. People read gritty murder novels largely because they want to be entertained, but also because they want to be a little shaken up. Scare them into not wanting to put the book down. Let them figure out the mystery as the book goes on, little by little. This is sure to keep audiences hooked.
Keep writing until it's done. Then, write some more. The art of writing is truly an organic experience, one that should never be rushed through. However, it is often helpful for many authors to get into a writing routine that they can stick to. It sometimes helps to have a goal to work towards, such as one chapter or a week or one page per day. For other authors, an entire novel can be written in one hungry weekend alone in a cabin. You really need to find what works best for you. Even if you set a deadline for yourself, do not be afraid to truly take the time to make it a piece you can be proud of. Don't stop writing and rewriting until the killer has taken his or her final life.
Edit, revise, and repeat. You may have written your final sentence, but that does not mean your book is done. Go over your work with a fine-tooth comb to be sure it is the strongest piece you can possibly offer. Check for smaller issues like spelling errors and grammatical mistakes. Most importantly, be honest in analyzing major plot holes and structural issues. If you can't be honest with yourself, ask someone else to take a look at it for you. It makes no sense to put in all of the work in writing a novel without putting in the effort to make it polished.