Do your market research. Find out what books are already available on the fifth grade topic about which you are going to write. Go to the library and review the material. Identify what is going to be different about your work from what is already available.
Research your publishing options. The three possibilities are to find a commercial publisher, self-publish or make the book available as an e-book. All three have their pros and cons, so weigh them before making a decision.
Put yourself in a fifth grader's place and ask what you would want to know about the topic. If it is a science book, for instance, identify the subject areas that would interest your readers. Check the curriculum guidelines in your state as this information will give you an indication of the background knowledge your readers will have on the topic and what you need to teach in the book at that level for students to meet state requirements.
Plan your schedule and set a deadline for completing the book. Be realistic about how much time you can devote to writing -- a full-time writer's schedule is much different from one who must squeeze the writing in between work and family commitments.
Outline your book. Figure out the chapters and subheadings you want to include. Aim for a word count. This will depend on the topic you choose.
Check online for free book templates. This Internet resource assists you in writing a book because it shows you how to organize your material into chapters. If you don't like what you find, design your own format.
Download the fifth-grade vocabulary list for your state. Print a hard copy of the list and then underline the words you will incorporate in your book. At the fifth grade level students are beginning to learn some Latin-based words, so be sure to include those in your book.
Find a free readability program from the Internet. These programs calculate grade-reading levels. Simply cut and paste your work into the program and it will tell you the grade level for the material, taking the guesswork out of writing. Check your work frequently -- every page or two -- to make sure you are writing to your fifth-grade target level.
Leave your draft for some time and then return to it when you are fresh so you can review it more objectively. Read your book aloud and assess it for tone, content and interest. Edit accordingly.
Ask a fifth grade teacher, or a friend or family member with good editing skills, to review your book. If you can't find anyone to do that, hire a professional editor. Even if you are a published writer, getting someone else to review your manuscript is essential.
Ask a fifth grade class -- your target audience -- to review your draft to get their feedback. Have them underline any words they don't know and check them against your fifth grade vocabulary list.