Casting a Circle
First sweep the area clean with your broom.  Visualize the broom sweeping away all negative energy.
When the area is clean, fill your mind with peace.
Then, build up power inside you.  Feel the energy grow until it's about to burst, then with your projective hand use your finger, athame, or wand to direct the energy from your body. 
Walk in a circle, clockwise, visualizing the energy flowing out of your wand and forming the sphere around you.
The circle is not flat.  It is a sphere.  You are standing on the ground, the exact middle plane of the sphere.  There is an equal halve of the sphere above you, and another in the earth below you. 
See the energy forming this sphere around you.
Usually the Circle is about 9 to 11 feet in diameter, with the altar in the center.
When you reach the North point again, the sphere is full.  Say,
"Circle grow and Circle flow,
as above, so below.
The Circle is cast. 
Not but love shall enter in
not but love shall grow within.
So mote it be!"
Now the Circle is cast
and you have created a sacred space in between the worlds.
Closing a Circle
Closing a Circle could also be called "opening a Circle" for that is more along the lines of what you are doing.  Rather than stopping the Circle, you are freeing or "opening" it.
So when you are finished performing your magick, first thank the Lord and Lady and the Elements.  Then walk counter-clockwise around the Circle, starting from the North.  Use your wand/athame/finger in your receptive hand and visualize the energy going into it.  See the Circle shrinking and diminishing into your wand. 
When you reach North again and the Circle is all taken up, say,
"Magick was sought for
and magick was wrought;
Up from the bottom,
Down from the top.
The Circle is open
but never broken;
Always it remains."
It is done. 
Now, if you wish, you may ground the energy, that is send into the Earth.
Or, if you wish, you may let it stay in the wand.
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