There are many ways to cast a circle. Some traditions create a circle and some create a sphere. Specific words and phrases may be used by individual traditions. Some people make up their own wording. Some people wing it and speak from the heart in the moment. Here is a basic no frills circle casting. Speak these words with a strong and sure voice, an authoritative voice:
“The Circle is now cast, and I am between the Worlds! Within these boundaries I am safe and no harm shall come to me!”
As you speak the words visualize a light coming from the middle of the earth up into your feet (this grounds you) and a light coming from the stars in outer space down into the top of your head (this connects you to spirit). See the lights meeting in your belly and then expanding outward to create a sphere around you.
When you want to disassemble the circle speak these words, again with much authority: The Circle is now open! I am fully returned to the Here and Now of planet Earth!
As you speak these words visualize the sphere around you dissolving as the light comes back into your belly, and each of the lights travels back to the stars through the top of your head or to the earths core through the bottom of your feet.
Now I will tell you why to cast a circle.
When you begin practicing witchcraft (or wicca, or energy work, or metaphysics) and start working with energy, your aura lights up like a little sunbeam shining into the darkness of outer space. This bright light catches the attention of folks on the astral planes( aka Otherworld, 4th dimension, etc). And not all of those beings are benevolent to you. Until you know more, and how to tell the difference, it is of the utmost importance that you are safe and protected during your adventures into the unknown. The lights create a boundary around you, and the speaking authoritatively creates a boundary around you.
Practice your circle casting daily. Sit for a few minutes daily in your circle. Make note of how it feels, what you hear, what you see(3rd dimensionally or astrally), how it smells. Meditate inside the circle. Or write in your journal, dance, listen to or make music, create art inside the circle. Try different wording and see if it changes your experience in the circle. Play with your circle casting! Have fun experimenting!