To train your shielding, you need to have creatures attacking you while wearing a shield (or a spellbook for higher level Sorcerers and Druids). Your shield can only block two creatures at maximum, so more than two will not help to train your shielding since the rest of the monsters are not blocked by the shield, therefore their attack is absorbed completely or not by your armor. All kind of hits are counted for shielding skill (blood, spark or smoke puffs).
While training, your shielding skill will advance optimally if you bleed your targets for any amount of damage once every 30 combat tries. A combat try is defined as either an attack made by or against you, thus with two creatures attacking you, it is necessary to bleed your target once every ten rounds for your shielding to advance.
Also note that you need a shield to increase your shielding skill. Using a weapon without a shield won't work.
Note that your character's total defense is only taken from your shield, or if you don't have a shield equipped, your weapon. If a shield is equipped while using a weapon, then the weapon's defense will be negated regardless of whether its defensive value is higher or lower than the shield's defensive value.
Some weapons have a defense modifier. If you use one of those weapons in combination with one shield, that defense modifier will be added or substrained to the total defense of your shield. For example, if you're wearing a Justice Seeker (Atk:47, Def:24 +3) with one Mastermind Shield (Def:37), your total defense will be 40.
Items with shielding skill bonus
There are items that provide bonus to the shielding skill. Those items are: