(Mouse movement)
Use a shield for some time to accustom yourself first to the idea of pressing the RMB when you see an incoming attack. It is the easiest way then just throwing yourself into the fray with a two hander. Once you can successfully block with a shield, boot up native. Open up the tutorial in native and go duel with the people there. Start with the settings on slow speeds and ask one of the duelists for a duel, not blocking practice. Once the duel starts, block as long as you can and never return a blow. Do this for about ~15 minutes before you start playing crpg. In a week or less you will be able to block effectively.
Later down your crpg career if you want to learn chambering, do this exact method again. Except this time, instead of blocking you are only allowed to defend yourself via chamber. In a week you'll be able to chamber like a god with that staff, but it works differently with the other weapons. For that you need to go to the duel server and practice chambering. However, learning to block and learning to chamber will not be everything in being a good player. Footwork is essential, and that you can only get by playing crpg more often. Watch how people move and turn into swings, or away from swings. Example you block right when the attack needed a left block, you can actually turn left and your right block will block the attack. Though, that is very advanced.
Regardless of the tips given by everyone here, the secret to blocking is...
Muscle Memory.
In a nutshell:
Use a shield to get used to noticing incoming attacks and reacting by pressing RMB. (If you want you can move your mouse in the proper direction here too, if it works you block, if it doesn't you block. Either way, you get more used to blocking.)
Boot up Native and duel the instructors in the tutorial for ~15 minutes every time you will play crpg.
Never attack them, only block until you die.
Repeat this for a week or two, or until you feel comfortable blocking.
If you want to learn to chamber later on, repeat this method.
Do not ignore footwork.
It will take time to learn, muscle memory is how you will block the most effective. Practice, practice, practice.