First of all one of my favourite tutorials for this is,
http://www.svartberg.com/tutorials/article_normalmaps/normalmaps.html
It's practically what I'll be doing here but will be breaking it down to it's core. If you want to find out more definitely check that out as well as the single greatest resource for understanding normal maps here,
http://wiki.polycount.com/NormalMap?action=show&redirect=Normal+Map#SGAHE
