So now you've done that research. You've put in the time and found some notes about that mythical creature you plan on using for that story of yours. But how much of those notes do you use?
The truth is, I don't have an answer for you. It all depends on you, and on your story. However, what I can offer is a way of filing through those notes to decide what is necessary.
I'm going to choose vampires as an example. I'm sure you've all heard of Dracula, the vampire that started it all. Now you have to decide for yourself how closely you'd like your vampire to resemble Bram Stoker's vampire. Let's face it, they probably won't be identical, not in a romance novel. Dracula, after all, was from horror novel. However, you could take certain things—such as the vampire's ability to shift into a bat or a wolf, or a vampire's inability to pass over running water—and build them in as the basis for your species. After you've done your research, this is the time you sit down and iron out all those kinks. Under usual circumstances, this shouldn't take you very long, because I'm sure if you're planning a novel about vampires, you already have some sort of an idea of how you want your vampire protagonist to think and act. As long as you have that idea in your head, you should be able to add traits to your list or reject them as you go along.
But what about the mythological figures themselves? After all, a myth had to come from somewhere, didn't it? Different authors have different ways of handling this. For instance, in Lynsay Sands' Argeneau series, she lists a deceased vampire's drunken ramble as the cause for the vampire myth. Or, like Maggie Shayne in Prince of Twilight, you could use the mythological figure itself, even as only a peripheral character in your romance.
Then again, there are those who build in famous poems like Der Vampir by Heinrich August Ossenfelder, either as the basis of the story, or recited at the beginning of the book to help get readers in the mood to read your book.
The key is to slip it into the story in a way that isn't detrimental for those who aren't well acquainted with the mythology. When you learn to do that, not only will your readers not be jolted out of the story itself, but those who pick up on the mythology will feel smart and gratified. A great way to do this is to feed it in in small doses. Think of it like a mystery—you don't give away the identity of the killer until the very end, but you do give enough clues throughout for your readers to make guesses of their own. If you sneak in these same clues and hints with the mythology as you go along, whether or not you have that big revealing scene at the end, just the fact that it's there will add another layer to your writing. Your story will be that much more compelling.