A Ritual is designed to aid the practitioner in guiding his/her personal energy towards a desired goal. In order for this to be at all effective, the practitioner needs to know how to connect with and manipulate this energy. The words and actions of a ritual are meaningless without this very basic, fundamental aspect. Needless to say, this takes practice.
With that said, Casting a Circle creates sacred space. It creates a place beyond the veils of normal space and time. It is a nexus of energy, a place of connection to the Source of all life. To Cast a Circle, you need to define the space, cleanse the space, then seal the space. How you actually do this depends on which tradition you choose to follow.
One method I've used is to use sage for defining the space, water to cleanse the space, and an athame or wand to seal the space. I walk around the circle the first time fanning sage smoke around the circle. (This is called "smudging") Then I walk around the circle a second time sprinkling water (This is called "Aspersion") while saying "I Bless this Circle with the Waters of Life" or some similar blessing. Then I walk around the circle a third time with my athame (sacred blade) or wand directing my personal energy around the edge of the circle, sealing it. Then, I go to the center of the circle and invoke the energy of Divine (howsoever you conceive Deity to be).
Other ways of Casting a Circle include specific invocations at each of the four quarters (East, South, West, North, in my trad). This is usually called "Calling the Quarters", or "Calling the WatchTowers". The purpose of this is to specifically invoke the all aspects of universal energy to enter the circle and lend support to the work to be done. In working groups, this process is often very ritualized and scripted. In open groups with varied memberships, the process of casting a circle will be more flexible.
~Flame RavenHawk
July 5, 2002