Backserving is a bit difficult to describe. This pictorial shows the techniques, but the photos may be too small to see clearly:

http://www.compoundbow.de/catfish_loop.htm

You backserve over the dental floss, then you put the tag end through the loop in the dental floss and then use the dental floss to pull the tag end through those last 10 wraps. Then simply pull tight and cut and burn the tag end.

Next time I tie one, I'll try to remember to take some close up photos. In the meantime, I'm hoping someone can come up with some better photos.