Shielding is done by many for many different reasons. You don't have to shield, persay, and some even believe that some people have natural shields that they don't have to cast themselves.
Some reasons to shield include:
-Spells/Rituals
-Scrying
-Sensitivity [Whether it be sensitivity to spirits and walking into a graveyard, sensitive to emotions, or anything]
-Comfort
There are also many different approaches to shielding, and many different visualizions, but the basic way that I learned is thus:
1. Draw up energy from the earth. You can imagine yourself as a tree, drawing up energy from your roots. [Some people imagine drawing energy from the air, through branches; Others draw energy from those around them]
2. Ball it up in your stomach area. Imagine just a small ball of glowing energy
3. If you're doing a spell or the like, imagine some of the ball dispersing and filling you with a glow of energy. You can use this to cast the spell later. If need be, replenish the ball with more energy. If you're not doing a spell or anything, and you're just shielding for comfort or something, skip this and go to step 4
4. Imagine the ball expanding into a hollow shell, a hollow sphere. Push it out until it surrounds you, making a bubble. If you imagine any dark energy [something like smoke, perhaps] , push it out of the bubble. Make sure you designate that the bubble is only to allow good energy in, and to push any bad out and keep it out
5. This is optional, but you can shrink your bubble to 'conform' to your shape. Imagine it shrinking, covering you kind of like saran wrap. Make sure it's thick over your head and chest
Now, you can use a visualization other than the bubble and the tree, of course. visualize ice for everything, even the shield being a kind of ice that can become liquid to move with Instead of shrinking the shield to form, push out that hollow shell until it fills my form, then push it out to lay on top of the skin.
To get rid of your shield, just think of it sucking back into you, then push it down as energy through your feet [or roots] back into the earth.
