There are many ways to cast a circle. My favorite and most used way is:
1. Always purify yourself first (like bathe in bath-water scented with a sacred or special herb, everyone's is different).
2. Wear something special that you only wear during a ritual, this sets apart rituals from everyday mayhem.
3. Plan out how big you would like your circle to be, which should be in front of your altar. Purify that space by sprinkling blessed water followed by blessed salt.
4. Meditate inside the area to be your circle.
5. Imagine a circle of light surrounding you (this light could be white, blue, or purple, purple is my favorite for it is identified with magick).
Circle the light around you, above, below, and all around in a clockwise direction. Feel the power of your circle, create it to your own tailoring.
Be sure and leave a doorway which your magick energy may exit. It should feel warm and peaceful within your circle.
6. Say your own personal chant and tell all evil that it is not allowed to enter your circle. You are safe within your circle if you make it so.
7. Invite the Goddess and God to attend your circle and ritual.
8. Relax and be happy. Preform your ritual when you are ready.