First, start by parting the hair. I have found that it works better with only half of the hair pulled up. And, if you use really wet hair for the buns, there are fewer flyaways and a better surprise!
Start twisting your piggy until it starts to double up on itself.
As you twist the hair, twist it around the elastic.
As you are twisting the hair around the elastic, twist it UNDER the previous hair. Keep in mind that the tighter your twist as you go, the better your surprise will be.
When you are done twisting, wrap an elastic around the Shreck bun at the very bottom so it catches in the end of the piggy. If you look close, you can see this on the left bun.
Here's a side view of the Shrek ear I just completed.