Elizabeth George suggests a series of steps which I am going to take the liberty of paraphrasing here:
Step 1: The Idea
Step 2: People the World
Step 3: Research
Step 4: Create Characters
Step 5: Create Settings
Step 6: Step outline till there's nothing more to say
Step 7: Plot outline
Step 8: Write the rough draft
Steps 9 through 14 - Fast reading, Editorial letter, Second draft, Cold reader, Third draft, Celebrate
There was an old Peanuts cartoon with Snoopy heading back to his dog house, thumbing through a dictionary saying, "Now I've got all the words. All I have to do is put them in the right order." How hard could it be?