Step One: Divide your detangled, second-day hair into two equal sections.
Step Two: Hold the righthand section in your right hand. With your left hand, take a thin strand of hair from the outside of the lefthand section, cross it over, and add it to the inside of the righthand section.
Step Three: Do the same thing on the other side: that is, hold the lefthand section in your left hand, use your right hand to take a thin strand of hair from the outside of the righthand section, cross it over, and add it to the inside of the lefthand section. You'll need to use gravity to your advantage as you keep changing hands, and if the strands keep slipping then your hair probably isn't 'dirty' enough (running some mousse or styling wax through your hair will fix this problem in a jiffy).
Step Four: Repeat the process until you've reached the ends of your hair, tie the braid with a hair elastic, and voila - you're done!
One of the most effective ways to wear a fishtail braid is on the side, as an extra-special version of the classic side braid. It not only looks awesome but means that you can do it yourself in front of the mirror, without needing to have anyone else there to help out. The looser you hold the strands, the 'fatter' your fishtail braid will be, giving a more relaxed finish to the overall look.
Rihanna, Amanda Seyfried, and Demi Moore keep their side fishtail braids relatively neat and tidy...
Other ways to wear a fishtail braid include as a single addition to loose hair (like Aubrey O'Day has done), in two super-girly pigtails (like Kim Kardashian's), or incorporated into a more elaborate braided style like Diane Kruger's (but unless you're a braiding expert, you'll probably want to leave this one to your hairdresser!)