Parry with a Weapon
When you are subject to an attack from your opponent, you may use your action to make an opposed attack roll with your weapon: if you beat their attack score, their attack misses. Remember, you only get 1 action in the round, so if you've already done something else on your turn, then you can't parry an attack. If you choose to parry, you don't make a defense check.
Parry with a Shield
If you choose to parry with your shield, you may apply the shield's tactic against your opponent if you successfully parry. For example, if you use a buckler and successfully parry an attack, you may use the tactic Disarm to force them to drop their weapon.
If you have the skill Athletics, add a +2 when parrying with a shield.
If you are wielding a shield, you can choose to sacrifice your shield in response to a successful attack. When you shield sacrifice, you negate the damage from a single attack, and the shield is destroyed. (You cannot use shield sacrifice if your shield is indestructible.) No roll is necessary to successfully shield sacrifice. Like any other attempt to parry, you can only shield sacrifice if you have not yet acted in the round.
Attacking with a Shield
Parry with Maleficence
When you parry with your maleficence, you spend 1 MP and make an opposed Smart check vs. the attack roll: if you beat their attack score, their attack misses. Whether you succeed in parrying the attack or not, you must expend MP.
In terms of the fiction, parrying with maleficence can look or function however you imagine. If you have a cold maleficence, for example, you may form a shield of ice that shatters when the attack is delivered, protecting you from harm; similarly, you might unleash a bolt of psychic energy to stun your opponent, canceling the attack. Any fictional positioning is allowed so long as the only mechanical result of the positioning is that the damage from the attack is negated.