Guess what? One can easily find lots of sites with directions for making four, five, eight, even 12-strand braids and fingerloop braids. But six? that's much harder to find, for some reason.
The first site I found with real directions was this one, and I wasn't sure that I could effectively execute that technique effectively with long strings (I figure I need two completed braids each of which is about 40 inches long--one for each side of the dress).
An amazing number of 6-strand braiding sites involve the making of challah, a kind of braided Jewish bread. Still, the directions given on this bread site appear to be both simple and effective, even if the pictures showing the braiding of glistening bread dough make me a bit queasy. :-)
Come to think of it, the pictures on the braided bread site look a *lot* like the sketches I've seen of the braid on the Hedeby piece (such as those are) and are so simple even I'm not likely to screw up
I may as well try the "bread" braid first. After all, what do I have to lose, other than a few feet of yarn and some time?
(15 minutes later) I now know the answer to my question; I could lose my mind. I've tried both sets of directions, and find them confusing even with the 12-inch strands I cut for a "test run."
Fortunately, I have a book about fingerloop braiding; I should look that up and see if it has directions that confuse me less.