Pick a person or an event you would like to dramatize. Go online and look up relevant facts about that person or event. Be sure that the websites you are on are credible in their information, otherwise you might end up including false information in your novel. Jot down this information either in your notebook, or save it in your word processor. Also, go to your local library and look at non-fiction books related to the topic. Gather as much information as you can before you begin writing.
Organize the information you gathered in step one. Organize by time and date of occurrence and also organize characters into groups that make sense to you. If you are able to collect any photographs on this topic, think of how you can incorporate the photo itself into the story.
Create additional characters for your story. They should fit in to the time period you are writing about. These characters shouldn't be your main character, but can be used to fill in gaps left by history, and possibly give your main character motivation. The characters you create should have little to no impact, however, on the actual historical event.
Create an outline for your story. Remember, the idea of non-fiction novels is that they are a dramatic retelling of the story. So, you will need to make sure the story leads to a climax in the end. Coming to a climax can be easy if you're writing about the Civil War, but it might be a little harder if you're writing about the life of Abraham Lincoln. Make sure your story is as historically accurate as possible in your outline.
Write the story based on both your research and your outline. When it is finished, set it aside for a couple of weeks. Then reread it and correct any grammar you need. While rereading, make any changes you can think to make the writing better, but not actually change the story. Put aside again and repeat.
Write a cover letter for the novel and start sending it out to agents.