The process of consciously surveying and taking inventory of the energy fields around us through whatever physical, biological or emotional markers available.
What are you feeling right now? Can you set aside all the events and situations that have caused you to feel the way you feel right now (or in some other high-tension situation) and take notice ONLY of what your body and mind is experiencing. This is very difficult for some of us who want to know exactly what event made us feel a certain way, but this process takes us away from analysis to action. That is not to say that you should not find out what or why something caused you to feel a certain way, but that Centering is the first step in taking control of how your physical (and non-physical) body functions.
Again as with Grounding, Centering should be performed before and after ritual workings and after any kind of psychic activity in which you direct energy from within yourself (or from outside sources) towards some goal. Whether this is through complicated ritual work, simple spell work or any other workings of Low Magick that you need to engage in when operating in the regular world, Centering will help you to function with less reliance on faith and more on what is really transpiring around you.
Actually, very few of us begin study with magick without asking at least once, "Is this all fake?" or "Am I just fantasizing that this is real?" Of course most people involved in magick eventually run across the concept that we live simultaneously in two worlds: the physical that we see, touch, smell, taste, and hear; and also the intangible energy world that many scholars like to call the Astral Plane. We are told have access to both planes though the Astral is the one we are expected to take on faith because we have no way of proving its existence.
Centering can help provide beginners with the "proof" that they are not imagining the energy world that parallels the physical one because they can identify definite changes in their mental, emotional and physical body as a result of energy fluctuations. And indeed this does not have to involve intensive and complicated rituals. Like Grounding, Centering (with practice) can be performed practically anywhere to help identify and remove excess or unwanted energies from your personal space. In fact, for those people who have some experience with Meditation, Centering is like a mini-Meditation session. It can improve mental and intellectual functioning, blood pressure, emotional health as well as a number of other health and well-being concerns.
The only real thing you need to get started is the ability to recognize what you body is experiencing. Once you can do this without any problems, you can move on to Grounding the excess energy and barring its return (Shielding). Creative Visualization is also helpful because you can use symbols to set the stage for making changes.
For example, emotions are very closely connected with the energy centers in our bodies so surveying how we feel (without dwelling on the "why") is the first step to connecting with this unseen area of our spiritual anatomy. You can survey how you feel after an argument with someone:
You notice that you feel stressed and even a little afraid (though most people refuse to allow themselves to acknowledge this--but it is instinctual and quite natural). This is a good start. Then you can account for your fast heart beat (all the while ignoring the voice in your head that wants to replay what the other individual said to you and you to him).
Once you have identified the emotions you are feeling, you can equate them to energy (perhaps a mist surrounding you and causing unpleasant sensations) and "color" that energy with whatever color, texture, scent, or feel that is appropriate to you. Once you have your body and mind's processes "labeled" with visualizations, you can move on to Grounding the energy accordingly.
Deep breathing during the centering process is critical. Many people overlook the profound effects of deep breathing and this is especially sad because scientists have recently uncovered that deep breathing is responsible for a number of healing effects on the body including improved immune system functioning and pain reduction. And, whenever, science endorses a technique known effective in the esoteric arts, we should pay close attention. Science hates admitting that anything having to do with magick could be effective and when it does, that can only mean it works. With the case of Centering, deep breathing can help you clear your head to more easily identify what you are feeling. It can also help force you body to take over control of energy currents that may have swung out of balance in the altercation thus putting your back in control of your system where you belong.
Once you have conquered the techniques of Grounding and Centering, you are ready to begin Shielding which bars harmful energies from returning. All these techniques used together form a very powerful system that makes energy currents tools you can use to your advantage instead of obstacles to overcome.