Research.... start anywhere, depending on how much you know already. If you need a background to start with, see what's on wiki and click their reference pages, then find things to look more in depth into. Google works. Books on mythic things exist...
As for writing, just write it the way you'd write anything else, or write how it occurs to you at the time. The actual act of writing doesn't exactly change. It might go slower if you spend time second-guessing yourself or making absolutely sure you've got your facts and whatnot right, and that's okay. Im my experience, though, just writing and seeing what happens works well enough. Unless it's something completely adverse to what you can reasonably do, I don't think you'll find too much to worry about. I purposely write things I see as adverse to my normal writing, just to see where I can take it. I usually do fine.
Going from vampires to elves doesn't strike me as something you'll absolutely have to find trouble with. Good luck.
