It's simple to initiate a block, but with everything that goes on around you and in a melee fight can make it difficult in certain situations.
Other than time, the best thing for me was how I orient my mouse and how to minimize how much I move my mouse when fighting. I had a bad habit of twisting my hand when trying to turn in game, and after 10-15 seconds my mouse was turned all the way to the left and my blocks started to mess up. You have to find a position that's comfortable for you and allows you to block easily. For me, I got accustomed to keeping my mouse pointed up, and adjust myself when it starts to turn too much. People know how to play where they mess up your timing and blocks (ex having to block right and turn left while moving your character, or block up/down and turn quickly, both tricky).
Another tip is to also learn how to keep pressure up with your own attacks. If you try to play too defensively and block all the time, you won't get much done. This can only be attained through practice, trial & error, and asking questions. It's very much a cognitive process where you keep track of what actions/strategies work for you. You also have to recognize when you try to attack too late after your block or other factors where you lose your attack phase. People say otherwise, but I use my eyes to block so I can retaliate as soon as possible.