As far as a diode recommendation I stand corrected, You want to use a Zener, not a Schottky. You need one rated at 6V and probably 5 watts. I could not find one on a quick search but this isn't my speciality.

Here's a link to a page that describes the process;
http://www.reuk.co.uk/What-is-a-Zener-Diode.htm

You may want to consider an off the shelf regulator
http://www.lynxmotion.com/Product.as...&CategoryID=48