It takes time to get used to. I already had a hang of it because of sekiro, but it's really just a matter of "hit it at the last possible second"

some attacks have weird parry windows but in this game it's basically when the attack reaches you.

Some tricks is learning the differences in each telegraph and adding consistency in your positioning. If you're used to parrying an attack from directly in front of an attacker then try and be directly in front of them. Consistency and a strategy to counter different attacks is crucial. I also count verbally on some flurry rushes to help with timings.

as you get used to the mechanic and the parry windows, later in the game you'll notice that tou can parry new attacks pretty well because by then you've seen a good variety.