Try starting here:http://www.gotopower.net/article.asp?art_id=65http://www.irf.com/technical-info/appnotes/an-1024.pdf
really all I did was google "smps flyback transformer design"
You need to pick a frequency,
then you need to determine the maximum primary voltage on your transformer
from this you can determine the max applied volt seconds applied to the transformer
then you need to balance the max flux density vs the primary turns on your transformer
there's more but that is the basic approach. the design of the transformer is somewhat iterative as you balance max flux, windings, cross sectional area, and winding area. You can't really go wrong with a BIG core to start with.
I don't usually do flybacks so you'll have to do some reading. I think you need a decent air gap in a flyback for energy storage. So you will probably have to put a spacer in between the cores.