Shielding is another basic skill, something done to help protect your Self from negativity and attack from outside your Self. It's very important to remember flexibility in this exercise - if your shields are rigid, you may find that it's hard to work with them later.
Centering and grounding is the first step to making a good, functional shield. If you don't know what that is, check out this page. Shielding is also very heavy on visualization - being able to "see", or image, with your mind the energy you are moving and shaping. Yes - you will be using your imagination. That's part of what visualization is.
Mirror shields are the most commonly-taught basic shielding technique I've seen online. I don't teach them.
Mirror shields are shields built like a mirror on the outside - slick, shiny, and anything sent towards you will just reflect off the surface of it Shields like this, build brick-solid, WILL take a pounding and not crumble - you will be very safe.
The downside of that is that they don't let all the negative things we do to ourselves out, and you can end up trapped behind a wall with nothing but your own fears and anxieties. Nothing goes out away from you, and no-one can get in to help you.
There's another problem with mirror shields. Any negativity cast at you from outside will bounce away, alright, but with no direction and may end up hitting someone else entirely, who may not appreciate that. That's how "witch wars" start, and it's in your best interest to avoid those.
So I don't teach mirror shields, because they're too easy to create badly.
This technique is something I call an aura shield, or perhaps more accurately a colour shield.
Center yourself, and then ground. Start by visualizing energy coming from your center to surround you. You are surrounding yourself in a field of energy that is both intense in colour, and light. It is as if you are creating a second aura around yourself. If you are aware of your aura, it should feel as if you are expanding and strengthening its intensity. You should feel, when you are creating the shield, as if you are "glowing" - you are projecting a very bright field of energy. Make it whatever colour pleases you most; I usually make it the colour of my aura, but a colour appropriate to your purpose is also a good idea.
While you are creating this field of energy, remember to visualize that negative energy is absorbed by the shield and sent down to the Earth to be grounded and cleansed. Also visualize positive energy coming to you through the shield. Positive energy should always be able to flow through your shields to you. Built the intensity as much as you desire in the shield, and then stop when you are comfortable with it. The shield will be there, even if you don't think about it at all later.
When you stop visualizing the shield creation and stop giving it energy from the Earth, it should maintain both colour and brightness. It will stay around you for as long as you wish it to - if you wish to get rid of it, just do the visualization in reverse and take it down. It's also a good idea to refresh the energy in it every couple of days, at least, if not every day. It won't need a lot of maintenance, but it's always good to keep it healthy.
After mastering this basic shielding technique, you may want to visualize yourself some custom shields. For example, some people like to visualize their shields as a constantly moving flow of water or fire - fire to burn away negative energy, or water to flow it down to the Earth Power again. If you don't like my techniques, play with them....you may end up making a shield that's completely unique and perfect for you.
If you'd like to go a step farther, and protect your private placesand sanctuaries against such things as negative energy, or scrying, or unauthorized entry, try creating a Ward.
Get grounded in the room or place you are warding, and pick a point to start. If it's a room, I start in the ceiling corners. Imagine a brightly glowing, burning ball of energy growing there - again, it might be the color of your aura, a favorite color, a mix of colours or one chosen for a specific purpose. It may also not be a ball, but a protective symbol you like; a pentacle, or other symbol, a rune or bindrune, or anything else you think is appropriate. These corner energy balls are the anchor points of your warding.
When you have anchored the corners, send energy out from one anchor point into the next, in a line - I visualize my lines as a braided cord of power, but yours might be a burning line of fire, or electrical crackling between corners. Whatever is most appropriate to your purpose is the best. These are the boundary lines - they define the boundary of your ward.
When you have finished setting the frameworks of your wards, meaning each corner has a strong anchoring point and each boundary line is strong, envision sheets of energy sliding down the wall. These should be flexible and strong, like the walls of your aura shield, and may be of any construction you desire. As you visualize the walls of the ward going up, give them intent - project your will for these walls as you would with your own personal shield. That can be protection from negative entities and energies, allowing in only positive energies, and keeping out uninvited visitors. Wards are as simple or as complex as you want to make them.