Casting a Circle
Cleanse your mind and picture the energy of the universe and nature mixed with your energy. Imagine yourself glowing purple or orange, whatever works for you. While facing North point your athame, wand, finger, or whatever you use towards the ground and see the energy flowing out. Begin to walk clockwise (right) around the circumference of the circle. Imagine a sphere forming with you inside it, half below the ground and half above.
Closing a Circle
Close the circle by picking up your athame, wand, finger, or whatever you used to cast it and walk to the corner at which you started. Point at the boundary of the circle and begin to walk counter-clockwise (left) until you complete the circle. While you're walking visualize the circle disappearing and the energy flowing back through the tool. When you are finished say "The circle is open, but unbroken."