Forget the people who are only telling you it just takes practice. Although that is true, that won't help you to do it soon enough.
What you need to do is get ready for the attack to hit then IMMEDIATELY after blocking hit forward+ hp + hk.
When I say immediately I mean before your character can even get the chance to start going through the whole block animation. That is what they mean by the window being small. I can guarantee you that if you practice doing this you'll get it with minimal effort. That's all there is to it, friend.