Fuzzy 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.

Maxx already gave a good answer for when to use Guard Impact so I'll focus on how to play without it.

This is coming from someone who played semi-seriously for months before starting to incorporate GI in SC4.

The first thing to know is your character's fastest moves. It's often just a straight horizontal hit. After that it's a matter of knowing your opponent's attacks and knowing when there's going to be an opening. There's always going to be some time interval either in between or at the end of a combo where you can get in a quick swing,by which I mean you can safely do attack, not necessarily that it will land unblocked. Even if your counter attack is blocked, at least now you're on the offensive.

I've only played SC5 once so far, but it seems like Just Guard gives you a slight edge where you might be able to get in a quick attack before the opponent has a chance to block in some cases, but it comes down to knowing your opponent's moves and how to react to them.

Also some moves have a higher tendency to supersede the opponent's attack than others. I know in general a vertical hit usually cancels out a horizontal hit. Some moves are also faster than the initially appear. I know personally I've found moves that I previously disregarded as not useful turned out to quite useful as they are faster than their animation would make it appear. I think it's an issue of our mind typically watching the time between the start and end of a swing, rather than the time between the start and the time of impact, which may happen well before the end of a swing.

Finally there's a rule of any pretty much any competitive game that has a meta-game component: Set up expectations, then break them. Repeat a pattern until the opponent figures it out, then break the pattern with something that counters what the opponent's going to do to counter you. For example, lots of characters usually have variations on combos, so there's mid-mid-low, repeat that a couple times, then occasionally throw in mid-mid-mid. They'll go to block the low and get slammed with the mid at the end (bonus points if it's a ring out!). It's like in Starcraft, making your opponent over-commit to anti-air, then coming in with a big ground army instead.