Hello Sara,
Thanks for the Tacitus (56 - 120) reference. If one goes to the Gutenberg Project's catalogue here,
http://www.gutenberg.org/catalog/ , and enters the word Germania in the title search, there are several books and translations online of his book. The downloaded text can be searched using CTRL F.
Two examples of the descriptions of the knot are:
The Oxford translation says: "It is a characteristic of this people to turn their hair sideways, and tie it beneath the poll in a knot. By this mark the Suevi are distinguished from the rest of the Germans; and the freemen of the Suevi from the slaves."
Thomas Gordon's translation says: "This people are remarkable for a peculiar custom, that of twisting their hair and binding it up in a knot. It is thus the Suevians are distinguished from the other Germans, thus the free Suevians from their slaves."
Unless the translators are lousy, I think it can be said that Tacitus is no help on how to tie that knot. He makes nice observations though!
On the google group forum, alt.religion.asatru, there is a thread on the Sueviian knot (Swabian Knot) starting on March 26, 2002.
In this thread, "Dirk Bruere" posts a link to pictures of the Swabian Knot:
http://freepages.genealogy.rootsweb.com/~vanrcwisner/histglob.html .
"Roo" posts a link to his best guess:
http://www.geocities.com/roo_two/swabianguess.html .
"Dirk Schmitt" posts this reference: "The Osprey Military Warrior Series book number 17 - Germanic Warrior 236-568AD by Simon MacDowall and Angus McBride depicts on Colour Plate A (after page 32), how to tie a Swabian Knot (Sueviian Knot). Its' ISBN is 1-85532-586-1." This book is still in print.
Which reminds me: I have a note somewhere to remind me to find that book ... hmm ...

I can not vouch for Schmitt's reference. Will you post an evaluation of this book when you find it?

Sara, with your research and post to this forum, I now understand why I occasionally see beautiful braids walk by me. Does your cosmetology college provide web space for you to show pictures of your hair designs?
I hope the above helps with your search.
Best wishes,
Brian.