Learn footwork, first.
Trying to learn how to manual block one of four possible attack directions is really, really difficult. If you have a decent grasp of movement and footwork, as well as the flow of melee combat, manual blocking becomes much easier. Still, much practice in the duel server is essential.
I played a 1H Shielder for about 6 months before I even tried to manual block. Then one day I picked up a two-handed sword in the duel server and just learned manual blocking. Having experience with a shielder taught me the fundamentals of movement, footwork, and melee combat flow, which made the process of learning how to manual block much easier.