Rule#7: Anticipate a Long Process. 
Historical novels usually take several years to write, as they require 
research at every turn. You won't always be able to anticipate what you'll 
need to know for a scene, and will constantly have to be returning to your 
references. This is entirely different from writing contemporary fiction.

Take, for example, in my part of the world, a trip from Austin, Texas to the 
nearby town of San Marcos. If you are going to write a present-day scene in 
which your character makes this trip, you will simply need to put him into a 
vehicle -- a pickup, or a Volvo -- and head him south for forty minutes on 
the flat terrain of interstate 35, passing strip malls and fields and the 
town of Buda. He will then take the exit marked "Wonder World", named for a 
local cave and visitor's center, and arrive in San Marcos. The only research 
needed to write this scene will be to drive the route yourself.

But if your character takes this journey in 1906, you will have to learn a 
few things before starting him out, and learn more things along the way. 
First of all, you need to know where the road is, and what's on either side 
of it, and what kind of conveyance your character is driving. If it's a 
flatbed wagon, what's pulling it -- a horse, a half-lame mule, two mules? 
How often do mules need water? How much traffic will there be? Any cars? 
What kind of food or luggage do you have along? And what if a wheel breaks, 
and you have to fix it, and you cut yourself with a rusty tool -- how do you 
disinfect the cut? Do you even know about disinfection? When did people 
figure out where tetanus came from? And -- assuming that you eventually make 
it to San Marcos, what's in San Marcos, anyway? As for the Wonder World 
exit -- when was the cave called "Wonder Cave" actually discovered?

But here is where the magic comes in: you begin to think, "Wow. The 
discovery of Wonder Cave. Now that would make a scene . . ." And then 
suddenly you have a story, and a book to write. The only problem, of course, 
is that you will soon find out that Wonder Cave was discovered in 1898 
instead of 1906, so you will have to move your story back eight years and 
find out what sort of vehicles they drove in 1898 and along what road, and 
the rest of it, or else joggle the facts and sacrifice credibility in the 
name of literary license. Or ditch Wonder Cave.

Writing historical fiction is like trying to get to San Marcos when you have 
no car, you don't know where the road is, and you have never in your life 
harnessed a half-lame mule to a flatbed wagon.

Assume it is going to be a while before you arrive.