Part your hair into two even sections.
Take a small strand of hair from underneath one of the sections and bring it over to the other side.
Take a small strand of hair from underneath the other section and bring it over to the first section.
Repeat steps 2 & 3 until all your hair is braided -- be sure to hold the braid tightly for each strand to make the fishtail effect more defined.
For a slightly messier look, pull a few strands around your face out of the braid.
For a more visual explanation, my video tutorial is here: