A Magic Circle is also a
circle of protection put in place before
spellcasting should be attempted, if the
spell is remotely
dangerous or could have
backlash. *Absolutely vital* in any
demon-summonings or
seances. (Although for the best
summonings, a
Summoning circle is a better bet.)
Can just be drawn in the air or on the ground with a finger, a charged stone, or a wand, but the best circles call the elements. Using dirt or salt for earth draw a circle large enough to sit in. Water can either be sprinkled/poured on the dirt (preferable) or small bowls/cups of water can be evenly spaced around the circle. Evenly space candles for fire and incense for air on the dirt circle.
At this point, if one is so inclined, an incantation can be said (although i dont personally believe in 'charms' or 'chants' so i never do). Alternately, just "charge" the circle, visualize positive, protective energy from the elements, being extruded and intertwining and protecting you.