The technique is fairly simple. All you have to do is envision a bubble, starting deep inside the subject. See it spreading outward until it surrounds the person. Then see this bubble shrinking down to the person’s skin, like a bodysuit.
Depending on what you want the shield to do, you need to customize the “look” and “feel” to be anything else. If you want it to reflect magick, it needs to be mirrored on the outside. If it is going to stop magick, the shield needs to be semi-permeable and flexible with a way to grab that Power as it impacts the Shield. Think of a net or something similar. To make Magick slide off, it needs to be totally smooth with no projections so the Magick can slide around it.
Make the shield as thick or as thin as you desire. The thicker, the more it stops and the less likely that it will shatter under an attack. You can set up two or more layers of shielding so the shields, in tandem, stop more than the sum of the parts.
If you want traps set (such as a trace spell to follow the path of the attacking Magick back to the caster) put them under the shield so that if it is penetrate, the trap goes off.
These can be set up to stop mental energy or psychic energy as well. This type of shield that is here is set for psychic energy, and a Magick shield can be set by using the Ritual of Warding. Psychic energy is from mind reading, and emotional emanations, and is really useful if you are at all empathic and are going into a high-emotion situation such as a funeral.
To make Magick shields, like I said, use the Ritual of Warding and shrink the area wards down to your body as described before.
Be aware, this will only stop new intrusions. It will not prevent something from affecting you if it is already there. You will have to get rid of the unwanted influence first, then cast the Shield to prevent it from affecting you more. And it will not prevent things that are generated from inside you. Those will be contained by the shield, and will not be able to escape. So know thyself.
Stars light your path.