Step 1: Materials (necessary and not-so-necessary)
We got through a fair bit of this. They come in large 8 foot by 2 foot sheets. They're meant for housing insulation but they also work great as the basis for lego heads. They also provide maximum fun while carrying them back for 20 blocks through a windy New York October. Depending on the proportions of your body size, you'll need to adjust your head size (and amount of polystyrene) accordingly.
Fans, Battery and Switch
All from radio shack - as I was building these heads I realized how ridiculously hot they were going to be inside. The fan was meant for a computer and the battery was a 9 volt with a small switch in between. If you're going for the less complicated route, this would be the thing to skip. In fact my better half was shocked that I was putting time and effort into such an elegant solution. Hey, it was worth it on the night!
Again, we tried to match an actual lego head as closely as possible. Home depot was our supplier in this case. Vast quantities of yellow and black (Groom) and yellow, white and black (Bride).