Im from the Dallas area so not as far south as you but pretty close. I have a really cool pair of box turtles (Rex is my male and Jas is my female). This will be Jas first winter with me but Ive had Rex a couple years now and he is a seasoned pro at hibertating at our house.
I have a simple outdoor enclosure that is a converted flower bed next to our house. What I do to prepare for hibertation is simple. I add one bag of potting soil in a mound that I pour on one side of the enclosure. I add a nice layer of leaves about 2 feet tall on top of the soil mound. Viola! That is it. Works great for me.
Oh, I will mention that here the turts will continue to feed until the first freeze. They will use the hibernation spot at night but come out mid morning and feed so I continue to provide food until they quit coming out. I do continue to provide water throughout the winter. Probably not necessary but I figure it doesn't hurt anything either.
Last year, when we had an extended warm period for a couple weeks in January, my turtles came out of the den for a few days. Was interesting to see them out in the middle of winter but hey that is the weird Texas weather for ya.
Good luck with the turts. IF you want to see a few pics of my enclosure and the turts, I have some on a website http://www.geocities.com/mkaarlela/boxturtle.html