Write one sentence that summarizes the idea of your book. This sentence should mention the main character, his conflict and the action he takes to resolve it, and what the consequence will be if he fails.
Write an outline for your book. This can be in any style that suits you, whether it's notecards with a description of each scene, or a chart complete with a timeline for the story. If you have ideas for subplots, you may include them. However, keep in mind that books aimed at sixth-graders are shorter than books for teens or adults and usually contain only one or two subplots. The story should stay focused primarily on your protagonist.
Write chapter by chapter based on your outline. Some writers prefer to edit heavily as they go, but this may slow you down, particularly if it is your first novel. Remember, you will be revising later, so try to get the story written first. Pay particular attention to the voice of your sixth-grade characters, and remember your audience. As an adult, it's easy to let a larger vocabulary or more formal phrasing slip into the dialogue. While you don't want to produce a novel filled with cliche slang, it's vital that your characters sound like authentic sixth-graders if children are expected to relate to them.
Set aside your completed first draft for at least one week. This may be difficult, but if you begin revising immediately, you may not be able to see all of the plot holes and poor phrases in the first draft.
Read through your book once, taking notes and highlighting places you wish to change. Make sure the pace is quick, as a sixth-grader's attention span is not as long as an adult's. While middle grade literature can contain beautiful prose and complex ideas, the writing must be accessible for a 12-year-old. Revise your book as necessary.