it's like any other game with parry mechanics. press space bar within the parry window. typically right before you get hit, the higher the difficulty the shorter the window.

no, the knife does need to be out, just press spacebar, but a knife does need to be equipped, and it can't be broken.

You can look for the little parry prompt in the lower right hand corner. and your timing doesn't have to be perfect unless you're on the hardest difficulty.

Simply time it right before the enemy is about to hit you, then pull out your knife to parry if timed correctly. Nonetheless, practice makes perfect!

yes

Pressing the knife button the near-instant an attack would have hit you makes a perfect parry.

The regular I think is pretty much the entire animation between pressing the knife button and Leon settling in to his Readied-Knife idle.