Doing so against friendly guards is great as they will only knock you out and not kill you. The best place to do this is by the river, North of the Woesong Bridge fast travel point on the White Orchard map (first town you enter in the game). Your timing needs to be perfect and it's going to take a few tries to complete. Always press the block-button when the enemy has his sword vertically in the air above his head (a fraction of a second after he starts swinging the sword and a fraction of a second before he hits you).
You must press the block-button ( ) just before the enemy is about to hit you. Look for a single city guard and counter 10 attacks in a row. Archers are also perfect as they always carry a sword. This works best against human city guards with swords.
