I was recently asked about how to practice guards for longsword, which brings up a series of related questions to answer the first one fully.
The most important part of training is to remember what your goals are, and how your actions help you accomplish those goals. Our goals in swordsmanship should always be 1) avoid death, and 2) eliminate the immediate threat to your safety. These two goals usually combine into the advice that is most generally useful; get your strong on their weak, and your weak aimed at their body.
A guard helps you to accomplish this by eliminating some potential opening attacks in the Zufechten, the opening phase of the fight, without creating additional openings. In other words they eliminate some of your opponent's options and do a little bit to simplify what you can expect.
There are two main ways that a guard will help close an angle of attack, the first is through creating a physical barrier such as in Ochs or Pflug. Your blade is aligned along one of the four diagonals such that any cut against you to that Peak (quarter of your body) will be funneled onto your strong, the part of your blade with the most amount of leverage, allowing you to trap their weak while keeping your own point free to attack them, thus accomplishing your goal.
The Second way that a guard helps you control your opponent's actions is through the threat of counter attack. The guards of Vom Tag and Alber use this by keeping your blade free of any contact until you opponent decides to move against you. In Vom Tag the essential aspects include keeping your hands close enough to the body that they do not become an easy target for a long ranged cut, and the point be back enough that you do not cock back before a cut. I cannot stress how important it is that you do not create unintentional openings for your opponent, especially during the early phases of training. Alber allows you to keep your point nearly fully extended and out of contact so that it cannot be knocked aside, allowing for you to aim your point at them in the time of the hand alone, while they will have to move in time of hand body and foot if you are keeping proper distance.
Now to address the original question: how do you practice the guards? Quite simply other than returning to them after a cut and experimenting to see what kinds of options you have from them, all you can really do is learn their names and make sure that you do them correctly each time you adopt one.
I don't especially advise changing guards a lot in the Zufechten unless you are well out of range of your opponent, as each time you change while within striking distance you create an opening for your opponent since it does nothing to threaten them or create a useful physical barrier.