Casting a circle is done for many reasons, and though it is always associated with Wicca, it need not necessarily be limited to this alone. A circle is cast as a containment, as a protection. Try to think of it as an invisible force creating a safe space to work in whatever that work may be. By creating a circle we are psychically sweeping that area clean of any negative influences or any inappropriate energies, and then creating a wall or boundary around it so that none might enter while you are working. The reason behind this is that when we invoke either healing-or good wishes-or whatever else we wish to invoke within a circle, If there is anything inappropriate present in that circle at the time, we may inadvertently have added it to our spell or our healing wish.
This obviously is not a good thing, so a little preparation whilst doing spell-work will repay you by making your invocations and spells clear and precise and safe. Before you actually cast the circle therefore, it is worthwhile standing in the centre of the space that you wish to clear, and mentally sweeping it with a large broom of white light of energy, so that it is sparkling and psychically clean! Whilst you do this say to yourself "that this space be cleared of all negativity and all inappropriate energies, and may any lost souls inhabiting it be returned to where they need to be." I use this little mantra before starting any work whether it be a meditation workshop, an invocation, or a workshop on any sort of psychic development.
To actually cast your circle, you need nothing other than your own forefinger (however if you have a beautiful crystal wand or something similar you may like to use this instead, providing it has been thoroughly cleansed and preferably offered previously to the Moon Goddess to bless!) Stand in the middle of your space-relax and focus your energy on the Crown Chakra. Now allow that it be opened and that pure white energy flow down on through this Chakra and down your right arm to your finger. Point your finger to where you would like your circle to begin, and then very slowly turn, drawing a large circle with the finger, keeping it even as you work around. It is well to describe this circle three times, to give your space protection and a firm barrier. (If by any chance you were doing this with a pet animal in the room, make a mental plea that a little doorway be left open for them to pass to and fro, but them alone!)
Your circle is now well made! Be sure that when you have finished your workings, you do a gathering up and windup your circle and do this in exactly the reverse of how you created it, taking the energy up with your forefinger but this time allowing it to travel down through your feet into the ground where it will be transmuted to renewed energy for the earth to use. This is an easy and wonderful way of creating a circle. Use it for whatever work you need to do. Its size can be small or huge, but to be careful when your casting a large circle that you do not impose on someone else's space- trapping someone else in your circle is not a good idea and can have very difficult repercussions!