The idea behind chamber blocking is that you block their attack while preparing yours, thus allowing you to attack them much faster than if you followed the normal block-attack-block-attack rhythm. It's a matter of timing. It's much harder to chamberblock a held attack, or an attack that's particularly fast or slow, or with weapons with a large length disparity.
Corrado's chamber pong is a good way to practice if you have a willing partner.
For me personally, I find that I have the correct chamber timing if I just left click where I might have right clicked for a block. As long as they don't hold the attack, it typically works out as a chamber. It's much easier to chamber overheads and thrusts, in my experience, than side attacks.
Also, when I play single player, I make it a specific goal in various AI fights to only chamber. Eg. In the Train the Peasants quest, I defeat the peasants by only using chambers. If you're just going for practice, it helps to set the damage you take as low as you can, so that you can afford to make more mistakes.
