The clever folks at ThinkGeek have put together a nice tutorial on how to knit a Princess Leia bun hat.
LOL, it looks great on dogs, too!
1. Get some comfy, Carrie-Fisher-esque brown yarn. Bulky, wooly yarn works best (especially for the optional felting step below) to make sure the braids are chunky enough, and you’ll probably need two skeins.
2. Procure circular or double pointed knitting needles. (Andrea used size 10 double pointed needles on her hat.) Grab a big ol’ tapestry needle while you’re at it; you’ll need it for the braids.
Read the whole tutorial on ThinkGeek!