There are many variations on how to cast a circle (also named calling corners). If you belong to a specific Witchcraft tradition, you should follow the methods of your own tradition. If you are an Eclectic Witch (picking the best of each tradition) or still exploring which tradition you want to join, then you can cast a circle with the basic elements and experiment with what works best for you.
Most circle castings call on the four elements (air, earth, fire, and water), once for each direction (north, east, west, and south). Some circle castings also call on a deity (one per direction, or sometimes your male God for two directions and your female Goddess for the other two, alternating between male and female with water and earth traditionally being associated with the female). The choice of which element to match with which direction varies by tradition. If you are casting the circle with others, each person can make the call for each corner or direction. You can experiment and each try different corners at different castings to see which fits each member of your coven best.
Your tradition may have specific sayings, ritual, or poems for calling each direction. You can make up your own poem or song.
The castings are done in a circle, typically clockwise in the northern hemisphere and counterclockwise in the southern hemisphere (although some Witches in the southern hemisphere also cast circles clockwise). Typically the castings start with the east corner.
Often the castings include ceremonially walking around the circle. Some castings will actually draw the circle (with chalk or sand), while others will mark it with incense smoke or candle flame. Some castings are entirely magickal (with the marking of the circle being a magick aura). You can combine methods.
Sometimes there are specific chants done while walking around the circle (a general chant of your Gods and Goddesss names will do some traditions have specific poems you can make up your own circle casting poem or song). Some Witches dance instead of walk. Often there are three passes around the circle (but only one calling for each direction).
If you use elements in calling each direction, you should also have a ritual that matches that element. Examples include lighting incense for air, lighting candles for fire, pouring water for water, and pouring dirt for earth. Your crystal can also be used as your earth symbol.
The ritual can be done at the corner or at the altar in the center of the circle (with a coven, it is common to have the high priestess and/or high priest perform the ritual at the altar while other coven members call the directions). If you perform the ritual at the corner, you will want to carry the element symbol (burning candle, etc.) around the circle while casting and then place them all on the altar before ending the casting of the circle.
If you are performing the casting solo, then you may need to carry the element symbol to the altar right away. This can be combined with the three times around the circle: cast the first element in the east, travel around the complete circle and place the element symbol on your altar, cast the second element in the south, travel around the complete circle and place the element symbol on your altar, cast the third element in the west, travel around the complete circle and place the element symbol on your altar, cast the fourth element in the north, and place the element symbol on your altar (going directly to the altar).
Many Witches end the casting with a ritual at the altar. Often this involves invoking the spirit of life as represented by salt (such as pouring salt). And usually this ends with a dedication or prayer to your deity (or deities). If the circle will be used for casting magick, then you might want to dedicate the circle to a God or Goddess associated with the kind of magick you will be performing. If the circle will be used for a holy day, then you will want to dedicate the circle to the Gods or Goddesses associated with that holy day. Of course, it is always appropriate to dedicate your circles to your personal God or Goddess (or both) for any purpose.
Your magick is done inside the circle. You should not cross the line of the circle except for emergencies. If you have to cross the line of the circle for an emergency, you should perform a ritual to repair the circle at your earliest convenience and before casting any new circle (the repair ritual does not have to be in the same location, although that is better).
When you are done, you need to ritually take down the circle. This can be simple or elaborate. Often it involves going three times in the opposite direction as the casting (usually counterclockwise). Often there is a thanking in each direction (instead of a calling) either thanking the element or thanking the deity (if you called on a deity for each direction).