I don't play Soul Calibur, so if any of this information is wrong or not relevant feel free to correct me
This sounds very similar to parrying in Street Fighter 3: 3rd Strike (although it was pressing forward at the correct moment, as opposed to pressing block(back in SF3)).
The only way to really get good at anything is purely through practice and experience. There are a number of factors to getting a parry (or just guard) right, including the attack animations speed, your own reflex speed, and whether you can correctly predict what is coming next.
With different moves having varying speeds, you'll need to learn when the appropriate time to just guard is. You'll only learn this by playing a variety of players and characters. It's not so much about academic knowledge itself as it is muscle memory. There really isn't time between when a move starts and when it hits to think about when to just guard, so it comes down to your ability to time the input based on your previous experience.
Playing casuals with friends is a great way to learn things like this, since the punishment for screwing it up is so high. Don't worry about losing too much against friends.
If you enter tournaments and aren't sure of your ability to just guard, act as though it doesn't exist until you've gotten the hang of it. It's not worth blowing the cash to practice something that you can practice for free elsewhere.
TL;DR unless there are certain input tricks (as I said, I don't play SC), the best way to get better is to practice with friends.