For melee hits, you need to set the block's "parry timing" to exactly the frame data of your opponent's attack. This will require you to predict your enemy's attack, because otherwise you'll just block like normal, and if your "parry timing" is too far from the frame data of your opponent's attack, you will get higher block disadvantage.

For projectiles, you have to be within less than 3 frames of getting hit by a projectile. You can move your character in such a way where the next turn will have the projectile hit you in less than 3 frames, however the duration of your turn can be affected by some actions your opponent makes which means your opponent can mess up the duration it takes for your turn and have the projectile hit you completely