How can I improve my ability to just guard?

The trick to doing just guard

To do the just guard, you have to press and let go of the guard button (for a total press of four frames or less) while your opponent is mid swing. You have to have already let go of the button before the attack connects to activate just guard. You have to let go of guard roughly 0.25 seconds before the hit connects.

The best way I've found to practice

Go to training and select a fairly slow character like Nightmare or Astaroth as your opponent. Set the CPU to counter after attack. The computer will automatically attack after each time you do. It's pretty easy to nail down the exact timing when you initiate it with your own attack. Attack, just guard, attack, just guard. After a while you'll be getting it on every try.

Practical timing in a live match

The three scenarios I've found the easiest to time against real opponents are:

1. Immediately after a guard. Many less experienced players will counterattack as soon as they're done blocking. If you practice using the method described above, it should be pretty easy to time when that counterattack is coming.

2. The last hit of an obvious combo. Most characters in the game have their trademark combos that a large portion of players use (usually B, B). Getting the timing between these two hits down is relatively easy. Guard the first hit and use just guard on the second. The timing should come naturally after some practice.

3. Unblockables. Probably the hardest of these three to time, but totally worth it if the people you play against favor using these. Unblockables are generally slow, so you have a second or two to prepare yourself and get the timing just right.

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.