Fishtail braids are nice because you're only working with two sections of hair.
Take a section of hair from the top of the head. Divide it into two equal parts. Take a small amount from the outside of the right section.
Cross it over and combine it into the middle of the left section. Repeat on the other side: take a small amount from the left side, cross it over, and combine it into the middle of the right section.
...Like so. Work with the hair in your hands until you start getting some shape. Once the braid starts to take shape, then you can incorporate more loose hair as you go.
Repeat the cross-over process, tightening up as you go.
Take slightly thicker sections of hair from the side that you want the braid to sweep away from.
Take slightly thinner pieces of hair from the side that you want the braid to sweep to. See how the braid is starting to shape to the left side?
Continue fishtailing for the entire length of hair.
Braid all the way to the ends. This is a great braid for layered hair because the layers will poke out, giving it a lived-in feel.
Now pinch the outside of several plaits and pull out to give it a lived-in feel and make the hair look thicker. Pin loose sections in place, and spray it with hair spray as needed.
And here you have it. Side braids are really in for Fall right now, and a fishtail makes it look even more special. It's great for showing off colored or layered hair.