Casting a circle protects you from any outside influences from other planes.
There are a lot of formal ways to begin a circle and some informal. I won’t be telling you HOW to cast a circle but I will link and refer to some of my favorite ways to cast a circle below.
This image is from my personal Book of Shadows.
Always begin by cleansing your space by mentally pushing out all negative energy with your besom. Then define your circle, this can be done many ways.
Stand facing east, in the center of the circle. Relax and breathe until you feel calm, centered, and present. Envision the wind whipping around you and get really in tune with the element of air. Simply say, “Spirits of Air, I call on you.”
Turn to the south. Envision crackling flames and the noonday sun. Get in tune with fire and say, “Spirits of Fire, I call on you.”
Turn to the west. Envision flowing water in waves and waterfalls and streams, possibly moving around your body. When you feel attuned with water, say, “Spirits of Water, I call on you.”
Turn to the north. Conjure up the scent of the earth after it rains. Imagine the silence and darkness of a cave and the rooted feeling of being barefoot on the earth. When you feel attuned, say, “Spirits of Earth, I call on you.”
Still facing north, become aware of your feet and send a column or roots of light deep into the core of the Earth. Bring golden white light up from the center of the earth and into your body. Say, “Mother Earth, I call on you.”
Then visualize a white light flowing creating the circle. To temporarily exit the circle, hold your athame in your hand and make a cutting motion across the line of the circle and say “I use this athame of crystal to open a door.” When returning, close the door.
To close a circle, thank the elements. “The circle is open, but never broken. The love of the Goddess is forever in my heart. Merry meet, and merry part, and merry meet again.”
Whitehurst, Tess. “How to Cast a Magical Circle in 6 Simple Steps.” Tess Whitehurst, 27 Nov. 2017, tesswhitehurst.com/how-and-why-to-cast-a-magical-circle-6-simple-steps/.
Wigington, Patti. “How to Cast a Circle for a Pagan Ritual.” ThoughtCo, May. 27, 2017, thoughtco.com/how-to-cast-a-circle-2562859.