Step #1: Formulate a concept. Know what the general idea of your book is going to be. Like a basic plot line, an idea of what needs to happen. Maybe even brainstorm a few events you want to occur at some point.
Step #2: Create and get to know your characters. Names, personalities, quirks; they’re all important.
Step #3: Write. And don’t stop. Create a simple roadmap for where your story needs to go. Then just write from point to point. Maybe you’ll have to skip around, but that’s ok. Just do NOT stop writing.
Step #4: Don’t get a second opinion, and only go back over it if you know you need to change something. Nothing slows progress like criticizing and editing.
Step #5: Don’t second guess yourself. If you like what you’re writing, keep writing it.