Medhue, master builder of Bento-based snakes and elephants and such has this tutorial on creating a Bento-based Second Life avatar head. Or maybe better to call it a pre-tutorial tutorial, since it's more about background you need to know before even starting. For instance, he explains, eyes to your Bento head need to be exactly lined up with the default eyes, or your eyes will look off. (Which might explain some odd eyeballs I've seen in Bento faces lately.) He goes so far to suggest getting rid of everything from the default head except the eyes.