Consider the following:

Why are you casting this circle? Are you casting this circle to commune with the Goddess (i.e. for a Full or New moon ceremony) or for a magickal working? A circle cast for simple communion with the Goddess does not need to be as carefully constructed as a circle used for magickal workings.
How many circles will you be casting? Traditionally, most circles are cast three times. Some people make one pass foe each face of the Goddess (Maiden, Mother, Crone), some do a pass each for cleansing, consecration, and empowerment. Some people feel one pass is sufficient. Some people just keep walking around the circle until they have said everything they want to say.
Who will you be calling to guard/watch your circle? You may have heard reference to Guardians and/or Watchtowers with regards to casting a circle. A circle is cast with reference to North, East, South and West. Because the circle is a place of power, some witches feel it is necessary to ask certain astral beings to guard the circle from outside influences. They go to each "quarter" of the circle (each direction) and call a being to guard that a corner. (See the chart below) Other times, you may be celebrating a festival and you won't need the protection from guardians or watchtowers. In this instance, you may want to invite these beings to join in your circle. This is done when your ritual is not going to have any magickal working take place and you just wish to celebrate with these elementals. It is worth noting that when you ask an elemental to guard your circle, they will be facing away from your circle (in order to guard it). Thuys, they cannot participate or aid you in any way. If you would rather the elementals particpate in your work, you should call them to join you, rather than guard you, in which case they can face your circle and add their energies to your work.

Once you have answered these questions,you have the beginning knowledge to cast your circle. Now let's talk about the structure of your circle.

Before you can cast a successful circle you need to understand exactly what you are supposed to be creating. We call it a circle, but really what you are casting is a sphere. You are casting a sphere of energy that goes up over your head and down into the ground. If it were just a two dimensional circle, it could not contain energy, which is the point of all this anyway. So when you are casting your circle, make sure that you understand and envision a sphere around you.

This brings me to my next point. Perhaps the most important tool for casting a strong circle is visualization. It sounds simple and easy but it can take practice to truly master. If you cannot see the circle as you are casting it (not with your physical eye but with your minds eye) you won't be able to cast a strong circle.

Back to the structure of the circle. The circle has four quarters as mentioned above. The circle is begun in the east and is cast clockwise so that you end up in the North. Whether you make one or three passes is up to you.

People often ask me if they need to mark the quarterss of their circle. This is personal preference. Since the circle does not exist on the physical plane some people find it useful to have markers set so that you don't accidentally walk outside your circle during ritual. But it is up to you. I place candles at my quarters.

Once you know the structure of the circle, you need to think of the energy you want to use to cast this circle. If it is a circle of protection, maybe you want to envision fire. If it is a circle for communion, you may want to cast a soft blue or soft pink glow. If you are celebrating, you may want to use bubbles, laughter, fireworks. Whatever suits you. As long as you use your personal energy and you can clearly see your energy, it should work well for your purposes.

The circle is cast by starting in the east, walking slowly clockwise around your circle, and envisioning your energy trailing out of your body (usually directed through your finger or you knife) and forming the sphere around you. Once your circle has been cast, you may walk to each direction and call your quarters (This is what it is called when you invite the guardians or watchtowers to join/guard your circle) It is after this is done that you will want to invite Deity into your circle.

Once your ritual is completed it will be necessary to take down your circle. There are many methods of doing this but the way I find easiest and still satisfying is to reach to the top of your sphere, "poke a hole" in it and collapse it down into the ground. (Imagine this: I stand on my toes, touch the top of the sphere and then pull the top of it down, kneel and touch the ground.) However you choose to do it, you will need to release your watchtowers, and then disassemble your circle. Do it in the opposite order of how you cast it. (In other words, the last thing you called to your circle should be the first thing you dismiss or release)

The energy you use for casting a circle needs to be focused. If you choose to speak while casting the circle (and most people do) you need to be sure your words are carefully chosen so that you are concentrating on the act of creating sacred space. The words you use to cast the circle should speak of your intent and your feelings. Keep your mind and spirit focused on the task, and visualize the energy pouring though you and forming a sphere around you, and with practice you will find success.