Blocking
Shields are made to block attacks, and while the base rules only implement this passively, a reactive implementation is highly intuitive. This implementation is provided here.
Blocking: Blocking is a new skill that uses Strength or Dexterity, shield wielders choice, modified by the Blocking Modifier of the wielded Shield.
Blocking Modifier: A usual shield has a Blocking Modifier of +4, with other variants being described in the chapter on Shields. To use the Blocking Modifier of a Shield, you have to have training with Shields.
Prerequisites: To use the features below, you need to be trained with Shields and wield a Shield.
Block: You can use a Reaction to try to block an attack that you can see and that targets you.
Make a Blocking check contesting the attack roll. If the result is at least as high as the attack roll, you successfully block the attack. It deals no damage.
When trying to block a critical hit, you roll with Disadvantage.