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?