! Simply put, a circle is a way to energetically create a boundary of sacred space to do your magical work. While it’s called casting a “circle” you are actually casting a sphere which has upper and lower boundaries above your head and below your feet. It’s like standing in the middle of a giant ball, keeping outside forces from disturbing your work and keeping your work contained until you have completed your ritual.
Casting a circle is very simple. Stand in the center of the room or area you’re going to work in. Take a deep breath and visualize a large amount of energy (whatever kind of energy that will facilitate the work you’re doing) and then use your wand, athame (ritual knife) or your finger to direct that energy in a circle. It’s often helpful to direct the circle line at the floor or ground just to keep yourself centered. Turn 360 degrees in a circle as you are “laying down” the line. Imagine the line moving up above your head and down below your feet to complete the sphere.
Practice casting. Add your own creation elements to the process. Maybe a chant or a song. Perhaps visualizing color or form. You will find your own way to solidify the sphere/circle as you become stronger at this skill. It’s important to do the work with your hands and your body instead of just intellectualizing the act. As you practice you will become accustomed to the magic and will be able to hear the voice of Spirit guiding you.
Some traditions believe that you should never walk through the walls of a circle once it’s cast. If this is the case in your tradition, and you need to leave the circle, you could have someone cut a door for you to exit/enter through. There are no strict rules, it’s dependent on your group’s beliefs. If in doubt, ask the person leading the ritual.
You will find that animals and children can easily move across the circle lines. I personally believe that cats and kids are inherently magical and that’s why this doesn’t disturb the circle.
Most trads will remove/undo the sacred boundaries when they are done but sometimes covens will leave things “up and running”.
A final note: not all groups use magic circles to create sacred space. Some covens will use a compass form or will work with ley lines to facilitate their space. Your mileage may vary.