The Source of Power
The first step to being able to use magic, is understanding how to absorb and process ambient energy into a usable form. Directing energy directly from The Stream is a dangerous thing to do, as (to reiterate) it's like putting crude oil in a car engine, it can have catastrophic, and almost always fatal effects. To get around this, we make use of the soul once more, except this time as a battery of sorts.
While the Aura and Soul passively filters energy, in Humans, the rate the soul uses energy, and the rate at which is can absorb at process it, typically breaks even, meaning the average Human won't develop a build up of energy, however, a Magi, with their unique connection to the PS Cells that reside within them, absorb more energy than their Souls use, meaning this excess energy builds up inside their Soul.
This build up of energy is referred to as one's "reserves", "reservoir" (or in more esoteric, archaic language, your "Mana Pool"). Though there is no set term for it, these are the more common ones, and you'd likely be able to use anything similar or with connotations akin to them and Magi will understand you.
There are two uses to being able to draw power from your own reserves, one is that you will always have a certain amount of power to wield, no matter the condition of the Ambient Field nearby to you, and the other is that this energy is already fully processed to match your Soul, allowing you to manipulate it easier than energy that's been freshly absorbed and may still be being altered; be wary though, each person can only hold a certain amount of energy as an upper limit, and too much magical exertion in too short a time will drain these reserves until you can't use any more magic without letting them refill.
Similarly, there are two primary way of "refilling" your reserves, the first, easier, and more obvious of the two is simply to wait and allow yourself to absorb more energy, assuming total magical exhaustion, the average Magi takes somewhere between one and three days to absorb enough energy to totally refill their reservoir.
Beyond simply waiting though, if one is to simply sit down, clear their mind, and willingly open themselves to The Stream, almost akin to a form of meditation, it's possible to accelerate the process, depending on the level of skill and natural power, from a matter of days to a matter of hours.
A Magi will always have a noticeable, natural connection to the Ambient Field, from the moment they're born. Up until they learn to draw energy, they will always feel as if they're missing something, a small part of them that's not quite whole. Magi typically learn to start drawing on energy during the latter part of their childhood, while on the verge of becoming an adolescent. It takes time and practice to learn, approximate three months of between half-an-hour to an hour of "meditation" each day before they're able to freely connect to the Ambient Field at will.
Only then, can a young Magi begin to learn Magic.
The Limit of Power
Each person can only store so much energy within them at an given time, which is one o the few limiting factors to magic. Say you wanted to conjure up a flame, little more than the fire atop the head of a match. This requires very little energy to do, and or most people can be sustained indefinitely. In turn, let's say that you, for whatever reason, wanted to make a nuclear explosion. This is something that would require the combined power of dozens if not hundreds of people, as in order to start it, you need split enough atoms in a small enough space, fast enough.
If a single person were to attempt it, then they may be able to make a few sparks in the air, but they would likely never be able to make a real explosion of that magnitude, no matter how long they try. Eventually (unsurprisingly quickly, actually, if they were trying to just start a thermonuclear reaction in mid air) they would run out of energy, as they would have been "using" (applying it as magic) faster than they were passively absorbing and processing it.
With your reserves empty, you would be in a state known as "exhaustion", or, to be technical, "Spiritual Fatigue", as your soul would have less energy that it would normally need to function properly.
The Abuse of Power
It was said earlier that one can exhaust themselves by overusing Magic, but it is possible to go steps beyond this, to the point where it can become dangerous, or even lethal. As Voltaire once said, "with great power, comes great responsibility", and sadly, this is true for Magic. In the last section, you discovered that you are able to use too much energy and leave yourself exhausted, but, there are consequences beyond this.
Repeated, often overuse of magic can have similar effects of over-excising. While regular practice, and a small amount of strain will cause one's abilities and control to develop and increase, f you go too far too often, it can leave you injured. with normal exercise, this could be a ripped muscle or a strained ligament, with magic, it can have physical, mental, and even spiritual effects.
In terms of damage to the body, due to it being the focus and director for the magic you wield, overuse of magic can leave you with muscle damage, where the energy has caused the muscle to move in ways they really shouldn't, as well as some more painful (but very hard to "achieve") effects, such as varying burns where there was either an uncontrolled surge of energy, or one decided to let loose too much power at once. These burns can effect both the outside of your body, and depending on the technique, can cause damage internally as well. A final effect to keep in note, is that due to the relation of PC Cells to the body, especially the Central Nervous System, overuse can cause nerve damage, which may lead to Brain Damage, pain, temporary or even permanent partial or whole paralysis, and in worse case scenarios, death.
The mental effects are, thankfully, less sever for almost every case. Mostly, overusing magic (in terms of the effects it has on your mind) will leave you feeling physically tired or lethargic, though this is mainly due to the physical effects. The worst mental effects are typically a headache or thinking slower, being unable to form more complex thoughts, etc, though these effects typically pass within a few hours. However, as with everything, there are exceptions. Using techniques that specifically deal with metal effects, such as telepathy, scrying, or taking attempts at Prophecy, can have a varying set of adverse effects, such as hallucinations, Temporary Psyonic Sensitivity, and varying degrees of madness.
The soul is the most difficult of the Tradiums to damage, meaning you either make a very, very small effect, or you cause an incredibly massive one. Typically, reaching Spiritual Fatigue means that your Soul has lost all of the excess energy within it, so your very being will have less energy in it, which will normally just leave you tired, in more extreme cases you may pass out for anywhere between a few seconds to a few hours, but you will recover eventually.
As a final note, if you enter a state of Spiritual Fatigue, is you have the willpower to actually stay awake without passing out to recover, it is entirely possible to carry on using Magic indefinitely. However, it comes with a drawback, a risk. Without a surplus of energy to wield, you are left with two options. Either, you draw energy directly from the Ambient Field, which would give you an almost literally limitless supply of power, or, you draw from the energy that makes up your very soul; both, are a very, very bad idea.
The former, if overused, will totally wipe your soul away and kill you. A similar effect stands for the latter, if you draw too much power from the core of your soul, then your soul won't physically have enough energy to hold itself together, and it will dissipate, killing you.
Of course though, those are only in the most extreme of cases, you'll usually just get tired and need some rest and food.
If you were learning to drive a car, you know not to drive into the river, similarly, you will always know when to stop. One need not let the follies of other deter them from knowledge, as long as you heed their warnings.