I am a fan of historical novels, both writing them and reading them.
Actually, that's not an entirely true statement. I am a fan of good historical novels, both writing them and reading them. Unfortunately, there are far too many examples of bad historicals that have put a bad taste in the mouth of many readers. I'm hoping to help you fix that with the same five steps I've used in writing historical novels.
1. Research. I'm guessing that, if you're interested in writing a historical novel, it is because of your immense love of history. However, I must warn you, loving history and a certain period or event in history isn't going to be enough for you to write a good historical novel. You have to know your history. And, much as I hate to say it, Wikipedia will only take you so far. You need to dig and dig and dig, like an archaeologist seeking to unearth an ancient civilization, trying to understand the period that you will be writing about. Read everything you can get your hands on, and read from every angle. Magazines, books, old newspaper clippings. Listen to the music of the times, collect the art, read other books from the same era. Immerse yourself into the world of the past and you will be able to successfully replicate it in your book. That brings us to the second step...
2. Build. Just like any other book you may write, world building is a very important prelude to actually writing your story. Don't let the fact that you will be writing in a real time and place, you are writing fiction and you'll be creating a world to put your characters in. It's important for you to establish the rules, know the mindset, and have the language in your back pocket. The greatest historical novels are ones where the author doesn't lecture the reader on the times, but (are you ready for this?) shows the reader the times through fantastic writing. Yeah, show don't tell. Who'd a thunk it would come up yet again.
3. Ask. In my opinion, every great novel begins with a question, and a historical is no different. In fact, historical novels may beg more questions than others. What would happen if the Lindbergh kidnapping was a meticulous cover-up? What if King Henry VIII was actually gay? What if a southern girl's father in the 1930s was forced to defend a black man accused of raping a white woman? (Yes, that's To Kill A Mockingbird, and I try to throw that in as often as possible.) You have to ask the right questions to get your story moving, and you have to ask the sort of questions that others may not have asked but will want to know the answer to.
4. Answer. I know there's always the argument of Pantsers vs Plotters, but I think that you have to have some element of plotting when you write a historical novel, simply because you don't have as much freedom to roam wherever you may please. You should probably know the end from the beginning and have a general idea of how to get there. I'm not saying you should necessarily do the snowflake method or anything (it works for some people, doesn't for others) but you should put some thought into it before you start.
5. Write the darn thing. Yeah, you have to put in a lot of work before hand, but it will pay off, I promise you. Just dig in and take all the foundations you've laid, and make a phenomenal piece of literature we can all be proud of. Easy right?