Did you all know that I got a bullet to the knee when I went to rescue George Washington from the Confederate Army when I got caught up fighting Hitler at Waterloo with William Wallace and Bonnie Prince Charlie at my back? No of course not, what did you really think?
The most important thing about writing historical novels is getting your facts straight. That doesn't mean you have to put all those facts into the actual book, for no one really likes to read a novel that sounds like a text book. But as long as YOU know the facts, it will make writing a lot easier and more effortless in the long run and people won't have to call you out on horrible mistakes.
The fun thing about historical novels is that you have a little leeway with what you do. Small inaccuracies can be explained in author's notes, and you can be sure there will be some, for somethings are never explained in history, thus causing you to have to come up with your own ideas. As long as you don't change major historical events to fit your story, no one is going to criticize you for small little tweakings. I for one, know how much research goes into a historical novel. There's a lot! But you'll find that it is much easier to write about something you know a lot about so research well before you write your book. That is the best advice I can give you on the subject.
It's fun to put your own characters into a historical novel and have them live what other people have lived, but it can also get out of hand if you don't know what to do with them. Say your character is at a famous battle like Waterloo. You can't have his change history and let Boney win (that's called an alternate history, and I will most likely address that in a later post) so you have to make sure you know what you're writing about. It never does to make stupid mistakes, (one of the worst is spelling and ethnic typos) and you will only feel like smacking your head for it later. Trust me, I've been there more than once.
You should always write what you know, but that is even more true for the historical novel. If you love the American Revolution and know the time period well, don't try to write a WWI novel just because you want to try it. Do lots of research first as it is very easy to tell when someone does not know about a subject. How can you tell? They try to spell out everything for the reader. They don't realize they are doing it, because in truth, they do it for themselves, but it is in no way advisable. No reader likes something spelled out for them. That doesn't mean you should make everything vague either. You'll find when you know a subject, you will be able to write about it with ease without too much description and obvious facts. Good historical novels let the story move and breath, they do not make it read like your high school text book.
So don't let Washington get captured by Hitler's Confederates at Waterloo. Get the facts straight and before you know it, you'll have one lovely historical novel to be proud of!