The following URL contains all most all of the Middle Pillar Ritual, except one fact.
In the middle pillar, you are supposed to visualise a black pillar to the North, that being the Pillar of Severity. To your left is the white Pillar of Mercy. You are the balancing Pillar between them, the Middle Pillar. You are representative of the Tree of Life.
This is best done after some sort of cleansing/banishment (I will be doing this following the LBRP I posted before).
http://www.feedback.nildram.co.uk/richardebbs/meditation/middlepillar.htmEnjoy, those who try it.
Edit: the guy also says to sit. I say stand unless you have something that makes you inable to do so. Sitting for this, in my opinion, diminishes it.