Fishtail braids take a bit of practice to get right, but the great thing about them is that you can wear them messy and they typically look great!
To make a fishtail braid, fasten your hair in a low ponytail. Split your hair into two sections, then pull a small amount of hair from the outside of one section and cross it over the the section to the middle (hold in place). Then, grab a small amount of hair from the outside of the ohter section and cross it over the that half of your hair AND the small section you just added to the middle (hold in place). Continue doing this until you have completed the fishtail braid.