Spontaneous magic is all about speed. You need practically no time to just let go. A few words, a gesture, a second or two and magical energy fills the air. What it does next, however, is everyone's guess.
When casting spontaneous, the wizard can set the Base and Intent as well as the factors Effect, Target and Duration - but everything else is up to chance.

And that is all there is to it. There is no way to control spontaneous magic. Once the energy is set free, anything (within the parameters designated by the wizard) can happen.
In game-mechanics term, you have to stop writing your roleplay at the point where you let some spontaneous magic happen. Then you have to wait for someone else to pick it up and write what exactly the magic does.
The magic will do roughly what you want it to do, but you have no control over the details. So if you are cornered by witchhunters and set free a Harm/Fire effect with Effect-4, Target-3, Duration-1 the result could be a Fireball. But it could also be setting the entire house on fire, or a stream of red-hot lava coming from your hands. Anything that matches all five elements of the effect can happen.

Like spells, the spontaneous magic you can do depends on your skill levels. Other than spells, however, there is no hard limit. As long as both your Intent and Base skill levels are equal to or higher than the power level of the effect you cast, the limits you imposed hold true. However, when you fall short of what you can control, that is when the real fun starts.
For every 5 points or frations thereof that your skills are short of the power level, something in the effect can shift. That means one point of Effect, Target or Duration moves to one of the other factors. Yes, there is conservation of magical energy.
For example, your Fireball-like spontaneous effect with your Harm 4 and Fire 5 skill levels falls a total of (12-4) + (12-5) = 8 + 7 = 15 levels short. That is three shifts. Shifts can be either within the effect (e.g. reduced effect, but on a larger target) or by discharging a fizzle.
A discharge results in the shifts creating a secondary effect with a power-level identical to the number of shifts moved into it. The side-effect must share the Base but can otherwise be anything and hit anyone. It often affects the wizard himself.
For spontaneous magic that results in more shifts than the power level of the intended effect, discharges are the only way to apply all the shifts.

Examples:
Instead of an Effect-4, Target-3, Duration-1 effect, internal shifts could result in many variations, for example if all three shifts come out of Effect it would reduce the damage to superficial burns (Effect-1), but it could hit a much larger group (Target-4) and keep burning for an hour (Duration-3).
The Fireball-like spontaneous effect could also spawn a fizzle. Three shifts could reduce the Target to one individual and the Effect from deadly to minor wounds, while causing any number of interesting Fire-based effects with a Power Level of 3. For example, a burnt hand for the wizard (Effect-3, Target-1, Duration-1). Or the air above his head being on fire for an hour (Effect-1, Target-1, Duration-3).
These effects can be combined. The same three shifts could cause one internal shift and discharge into a fizzle with a Power Level of 2.

Obviously, spontaneous magic far above your skill levels is a really dangerous idea. Many of the legends about the evil and dangers of magic have their cause in spontaneous effects gone horribly wrong.

One advantage of spontaneous magic is that since you excerpt less control over the effect, it only takes half the concentration of a comparable spell.