There are dozens of books on the market, and hundreds of websites, that detail the tools necessary for casting a circle, as well as how to cast the circle with those tools. Very few tell people that the only tool they really need is themselves.
Being able to cast a circle without any tools is a very handy skill. It allows one to cast a circle anywhere, any time, under any circumstance.
There are many methods that someone could use to cast a circle without tools. Some simply have you use your hands in place of the Athame or Wand.
The method below requires no movement (though you can move if you want too), and can even be done in the midst of a crowd of people without them realizing just what it is you're doing. This is very useful if you begin to feel overwhelmed; you can cast the circle to shield yourself from excessive energy. Once you are comfortable with this technique, you'll be able to cast your circle at a moment's notice.
Begin by taking a moment to ground and center yourself. Focus on your internal center. This is the area roughly behind and slightly below your naval, or belly button. You need to find your own center, so take just a moment to do so. This may take some practice, so be patient with yourself. While centering, breath in through your nose, and out through your mouth.
Once you've found your internal center, visualize a small ball of energy, about the size of a golf ball, forming at your center. Visualize a small stream of energy coming up from the Earth to feed this ball. The color of this ball can vary depending on your preferences, and on the purpose of the circle. For general use, and for practice, use a white ball, which holds all of the colors. Here are some basic color meanings; you may find different meanings elsewhere:
banishing, protection, shielding
passion, energy, sexuality
healing, wealth, fertility, abundance
psychic ability, spirituality, connection with the divine
Remember, these are just basic correspondences, you should use what works for you.
Once you have the small ball clearly visualized, begin to expand it. You'll want to do this slowly at first, too fast and you'll lose it. As you breathe out through your mouth, allow the ball to expand; as you breathe in through your nose, allow it to rest at that size. Continue to do this until the ball is begins to move outside of your body.
At this point, you may encounter some resistance (or you may not!) That's okay. Take a few breaths without expanding the ball, and then begin the expansion again. If at any point you feel resistance, take this same pause, breathing and allowing the energy to settle for a moment.
Continue to expand the ball until it is the size you need it to be. If you are casting the circle for a large ritual, then you will, of course, want a large ball. If you are casting for a mediation exercise, you may only want your ball to reach out enough to encompass your entire body.
When the ball, now a large sphere, has reached your desired size, stop expanding. Continue to breath steadily, in through your and out through your mouth. Visualize the flow of energy from the Earth begin to subside, and then stopping entirely. Visualize the sphere being sealed, so that no energy escapes. Take a moment to visualize the sphere become more solid, and state, either out loud, or in your mind, the intention that the sphere stay in place until you are ready to release.
Now you can begin your intended work.
When you are done, you will want to release the sphere. The process to do so is essentially the reverse of the casting process. Begin by focusing for a moment on your breathing; in through your nose and out through your mouth. Re-establish the connection between the sphere and the Earth. Visualize the energy of the sphere beginning to flow back into the Earth. As you breathe in, visualize the sphere contracting, getting smaller with each breath. Allow the sphere to rest with each exhalation. Continue this until the sphere is back to its starting size, and then visualize the sphere dissipating entirely. Ground and center yourself, and then go on your way.