In order to dismiss the circle, you would go around the circle the opposite way from which you cast it. Since you will most likely cast the circle going deosil (clockwise), you would dismiss the circle going widdershins (counterclockwise). To dismiss the quarters, you would walk around the circle's perimeter with the tool you used to cast the circle, visualizing the energy being sucked up back into the tool. You can dissipate it by dismissing it: "The circle is open but unbroken. So mote it be."

There are several other methods of ending the circle. You can visualize the circle sinking into the ground, or visualize it dissolving and filling the area with warm, loving energy. Research. Experiment until you find the one that works best for you.