Just like anything else on the pagan path, casting a circle is very individualized. Some prefer an in depth ritual. I tend to visualize my self protected by a white light - from both inside and out - that is connected to the universe. In this way I feel that I am always protected by my circle.
Here is a very simple way to cast a circle, but again can be personalized. For instance the wording or "chant" can be completely written by you...some tend to use each element....air=breath, fire=spirit, water=blood, earth=body. This is basically calling on nature or goddes to protect you.
Close your eyes, and imagine a tiny light in the center of your heart chakra. The light is pure white and every time you exhale it grows larger and larger until it fills the space (above and below like a real sphere) of your designated circle. When you have finished filling your sphere, recite the following:
Yea, This Ground Of Mortal Clay Nigh Raise the Gates To Windows Fae
Use either your RIGHT index finger, a wand or the tip of an athame blade, to define the area around your circle and bind the energy within. This action also bars any unwanted entities from entering the circle while you are working. Always begin and end at the eastern most point of the circle (the direction of the rising sun) and also make sure to travel in a Deosil direction (clockwise).
Once your circle is securely in place, carry on with any additional ritual workings you have planned. When you have completed your ritual session, simply re-define the circle, once again beginning with the east, but this time move in a Widdershins (counter-clockwise) direction.
This closes the circle and allows the remaining un-used energy to disperse. Make sure to Ground and Center yourself after you have closed the circle.