Fingerloop braids are made by cutting equal lengths of thread and attaching both the ends (usually knotted altogether) to a fixed point (doorknob is good - coffee-table leg also works) to create a set of loops. The fingers are then inserted individually into the loops ceated, and each loop is pulled through the others in the style as set out in the pattern.
Braids are either flat in cross-section or half-ellipse (flat on one side and rounded on the other). The former was used for edging, poynts and seal tags, the latter was perfect for couching - giving a nice flat surface for the side you place on the fabric, and a raised section for the decorative surface.
It is generally quite difficult to make fingerloop braids to any great length, as you are limited by the length that one can comfortably reach.
The techniques of fingerloop braiding are discussed at length in "Compleat Anachronist #108" available here: https://secure.sca.org/cgi-bin/stockclerk/ca.html
a basic introduction to the technique is located at (although learning in person in generally quicker):
Pictures of some fancy braids (no instructions) can be found at: http://www.cs.vassar.edu/~capriest/fingerloop.html