Step 1: Separate the tail into three even sections. If they're at all uneven the braid will look bad.
Step 2: Braid each section individually. These braids also need to be even.
Step 3: Braid the three braids together. Now this is the part that makes it pretty and the part that's hard to explain... Basically, the braid has to be thin and flat. You don't want it to overlap too much, because you want to be able to see the three braids so it looks intricate. I hope that makes sense.
If the loose hair at the end of the braid looks funny, you can fold it up under the braid and put electrical tape around it.
Here are some pictures, and sorry about the quality - they were taken with my cell phone:
http://i48.tinypic.com/35cldp5.jpg (this is after my ride so it looks a little ratty)