Learning
There are really only two ways one can learn Shielding. The first is through being taught by an Artisan ranked Shield Mage (if you are an Artisan+ in a different magic) or by a Master+ ranked Shield Mage if you have no, or lesser, magical experience.
The other method is through an intense emotional desire to protect someone or something. This can often appear suddenly in the midst of strife and danger.
Forming
Shield Mages can create two types of Shields. The first, and most common, are Stand Alone Shields. These are formed when the mage exudes their magical force through their skin, typically through their hands, though not required, then shape the Shield using their mental control of it. Early on, most Shield Mages struggle to make stand alone shapes, so they often apply the Shields as a second skin to an existing object, person, etc. This type of Shielding uses a good deal of the Shield Mage's life force, but once it is created, will not use any more.
The other type are Instant Cast Shields. These Shields are often weaker, but can be conjured up instantly around the Shield Mage's body. These have a very high up front cost, as well as continually needing to be fueled.
Tasking
Tasking is what gives Shields' their strength. A Shield can be tasked to Repel, Absorb, Emit, or any combination of these things. Repel allows a Shield to prevent a specific substance or individual to pass through the Shield. However, these tasks must be very specific in order to be stronger. Tasks that are general and vague in nature will often be weaker, as their power is distributed evenly against possible sources.
Example: A Shield spread across a doorway is tasked to Repel sentient beings. Since there are a large amount of sentient beings, and differing types, it may only take a few attempts for one to break through the Shield.
If that same Shield were tasked to Repel Humans, it would then require many more attempts for them to break through.
Finally, if that Shield is tasked to Repel Bob Durden, it is very unlikely Bob will ever get through the Shield.
The Absorb task can only be applied to energy or other magicks. You cannot task a Shield to absorb a human, but you could task it to absorb kinetic energy from being struck, or sunlight, and that energy would be stored in order to maintain the Shield, making it stronger and last longer. That stored energy can also be used for the Emit Tasks.
The Emit task can only be applied if a Shield has an Absorb task in place. The task allows the Shield to Emit energy, in a wide variety of forms, determined at creation by the Shield Mage. If a Shield is tasked to Absorb sunlight, and emit heat energy when touched by a human, then when a human touches it, the will receive the entirety of the Absorbed Sunlight converted into pure heat.
Layering
A Shield Mage can Layer multiple shields over one another, for increased protection and function. This must be done from the inside out, so the inner most Shield must be created before any of the outer layers. Each of these Shields can have their own tasks, but they are applied in the order of the Shields' order
Using the same example above, 2 Shields are formed across a doorway. The innermost Shield is tasked to Repel anyone named Bob. The outermost Shield is tasked to repel leather. So if Bob Durden, wearing leather armor, attempted to pass through the doorway, the outermost Shield would repel his leather. If he managed to attempt it enough to pass through that outermost Shield, the innermost would still repel him for being named Bob.
However, if Steve Durden, wearing only cotton clothing attempted to pass through the doorway, he'd find himself entirely unhindered.
Repairing and Unshielding
Any Shield Mage can repair their own Shields by applying more of their magical force to them, and can even alter or remove the tasks set therein. A Shield Mage can only repair the Shields of a different Shield Mage if they are of a lesser tier, or are there to allow permission. They can also remove a shield, and if done carefully, can reabsorb the remaining magical force in it.
Bob Durden is a Master Shield Mage and has found a Shield that was created by Artisan Shield Mage Steve Durden. Bob could repair and strengthen this Shield, alter the tasks, without Steve knowing.
If Steve found one of Bob's Shields though, he'd find he'd be unable to do anything to them.
Group Shields
Shields can be created by multiple Shield Mages. This allows for larger Shields to be formed with the magical cost being spread evenly through the group. The amount of tasks is limited to the highest Tiered Shield Mage in the group though. Group shields are not necessarily stronger than an individual Shield, but simply larger in size.
Side Effects and Dangers
As with all Magicks, Shielding has varying, negative side effects and dangers. All magic is addictive, and can often lead mages to using far too much magic. Doing so can cause anything from overexertion, to passing out, to loss of magic temporarily or permanently, to death.
If a Shield Mage is attempt to form a Stand-Alone Shield beyond their capabilities, they will find that it will fail, and all the magic spent will be lost. On rare occurrences, the Shield might rebound and form itself around the Shield Mage, which can be inconvenient, painful, or deadly, depending on the Tasks it was set with. If a Shield mage has an Instant-Cast Shield that is receiving too much damage or energy from an outside source, they run the risk of complete magical depletion if they do not Unshield.
Progression Summary
Novice - Can make Stand-Alone Shields roughly the size of a human adult, and can only assign 1 task per shield. Their Instant-Cast Shields will be roughly the size of a dinner plate and are incredibly unreliable, failing more often than not. Novice Shield Mages do not have the control or knowledge to Layer Shields, to participate in Group Shields, to Repair or Unshield, nor can they teach.
Journeyman - Can make Stand-Alone Shields roughly the size of a room, and can assign up to 3 tasks per Shield. Their Instant-Cast Shields are roughly the size of an adult human. They have the ability to Layer Shields and participate in Group Shields. They cannot Repair or Unshield, nor can they teach.
Artisan - Can make Stand-Alone Shields roughly the size of a building, and can assign up to 5 tasks per Shield. Their Instant-Cast Shields are roughly the size of a room. They have the ability to Layer Shields, participate in Group Shields, and can both Repair and Unshield. They can also teach other Artisan+ ranked mages of other disciplines.
Master - Can make Stand-Alone Shields roughly the size of a village, and can assign up to 7 tasks per Shield. Their Instant-Cast Shields are roughly the size of a building. They are able to perform all abilities of Shielding, and can teach anyone Shielding, regardless of magical knowledge.
Grand Master - Can make Stand-Alone Shields roughly the size of a city, and can assign up to 10 tasks per Shield. Their Instant-Cast Shields are roughly the size of a village. They are able to perform all abilities of Shielding, and can teach anyone.