Why the details matter
People will notice. Your readers are not stupid. Maybe someone else's are, but yours are not. Good agents and editors will notice the lack of accuracy and send you one of those form rejections that have been photocopied so many times they're blurry and off center. More important, if you don't care about the details you will not get published anytime soon because the only way to become a writer good enough to get published is to care about the details.
Historical Romance. Honestly, how hard can it be?
Throw in some long skirts (they swish around your heroine's ankles) a few immaculate cravats (often intricately tied), some breeches and a couple of "bloody hells!" (uttered with a crisp accent) and voila! You've got an historical romance. Sitting in the bottom of your desk drawer. Not convinced? Read this.
Ferret out or otherwise hunt down the odd fact you need to confirm. But keep an eye out for the well-reviewed book on your period of interest. Never hurts to read a fresh point of view.
OK, so what do I need to do?
Research. My advice is pick the time period you like best and read all about it. You must know what was going on socially, politically, artistically, architecturally, and every other ally there is. Yes. That's a lot of reading and with one exception, no, you absolutely cannot use a work of fiction for your research. You have to read history books. That's plural for a reason. You'll have to get your hands on books about clothes and architecture, furniture, and carriages. You'll probably need to read about ships and weapons and money. Contemporary diaries are really really nice if you can find them. You might want to check the resources section of this site.
The good news is that once you've done the basic research, you only need to refresh now and again. Then ferret out or otherwise hunt down the odd fact you need to confirm. But keep an eye out for the well-reviewed book on your period of interest. Never hurts to read a fresh point of view.
What's the exception?
Contemporary fiction written and set during the time period you've chosen. Read and absorb. You'll find you'll end up with a feeling for the people who lived in those times. However! This is not a substitute for confirming your facts.
If you like the English Regency, then read Jane Austen. Actually, read Jane Austen anyway. Read the Brontë sisters, and not just Jane Eyre and Wuthering Heights. Vilette is a brilliant, brilliant novel. Captain Frederick Marryat wrote some wonderful novels that take place during the Napoleonic war. As it happens, Captain Marryat was in the Napoleonic War. If you want to absorb the flavor of the era (and you should) read Captain Marryat. Do you like the 18th Century? Read Smollet and Richardson, Voltaire and Rousseau. Victorian is for you? Charles Dickens. The lady who wrote the Semi-Attached Couple and the Semi-Detached House. Horrible but instructive in attitudes.
Horrors! What if I become well-read?
It can only help.