Create major characters. Most importantly, start with the hero and antagonist. Your hero should have a complete bio; ideally, so should the antagonist. The more you know, the easier the writing will be. Other important characters can be created by considering questions such as who helps the hero (or antagonist) and why, who means the most to the hero and who is the person who most hates the hero (this may not be the antagonist).
Decide the story's major conflict, which the hero will spend the story trying to resolve. There will be more than one conflict and these will provide subplots, but the major conflict is the engine of the story.
Decide the beginning of the story, where the hero experiences some event that propels him onto the path of the story's plot. Decide the presumable ending point--presumable because, as the story is plotted and written, a better direction and destination sometimes emerges.
Write down the major events of the story, those events and situations you absolutely know will happen. You can think of these events as destinations, the story's path plotted between them. There will also be many minor stops that the story winds through, sometimes, it seems, without your say-so. These smaller destinations keep the story moving forward through each major event.
Decide how many words the story will have. Shoot for something manageable, but at least 50,000 words. Divide the words by 30 days and you have how many words you must write every day to reach your target.
Choose the month you'll write the book.
Write the book, day by day, hitting each daily word count target. Start at the beginning, heading for each major event you've already planned. Don't worry if the story begins to veer in unexpected directions, but do give immediate thought to how you'll reach your major plot destinations given the new turns of events. If you discover the story has changed from the original you intended to write, repeat the major events planning immediately, while still reaching the daily word target by writing scenes you are still sure will be in the story.