A few years ago, I made my nieces Star Wars costumes. They come by their love for Star Wars genetically. Their parents wedding recessional was the Star Wars theme played on strings.
Sadly, there really are only 2 female characters in all 6 movies. The real reason why I wanted to make this costume is simple. That hat. It kills me.
The rest of the "costume," if you want to call it that, isn't much. It's all about the hat. Princess Leia's coif is so distinctive that it's really all you need. You could wear that hat with jeans and a t shirt and everyone would instantly know who you are. And she might.
Princess Leia Hat Pattern
Brown yarn (I used Vanna White yarn from Michaels)
White turtleneck onesie
White fabric (1/2 yard or so)
1/2 yard 1/4" elastic