As a reader, I like pretty much any magical system, as long as it makes sense within the story (read: no deus ex machina spells, if you said that something is impossible and even based a good part of your story on the premise that a certain type of spell is impossible, then you better stick to your own rules - looking at you, Once Upon A Time XD). As for cool ways to portray magic, I found what they did in The Magicians -with the weird hand gestures- to be very intriguing... though I only saw a few episodes and I can't really judge on the magic system of the series as a whole.
For my comic, the main idea is that magic is more or less a skill that anyone can use. Like with any other skill, there will be people that are naturally better at it and others that struggle, but the key is that anyone can learn, as long as they put effort into it. There is a genetic component to magic, which is why magical bloodlines are a thing, and usually mages born within a certain bloodline tend to be more powerful than others, but that doesn't mean that they're the only ones who can do it: they just have it easier than others.
Magic can be pretty taxing, both physically and mentally, so if you're stressed or tired you may want to refrain from doing it, otherwise results can be rather... unpredictable. Most witches specialize in one or two branches of magic, usually starting with elemental magic, since it's the easiest to work with it. Other types of magic, like shapeshifting, can be VERY complicated and only a few witches/wizard are capable of doing it, with limitations. In some cases, mages can be naturally gifted with a special ability (mind reading, precognition, whatever)... though, depending on the case, that may not always be a good thing D: in short: magic cannot be used for everything and if you don't know how to use it, it will backfire.
As far as how spells go, there are a bunch of ways: you can move your hands or snap your fingers and produce flames/water/wind, but it will usually only last a few seconds. For more lasting results, you may want to work with an already existing elemental source... and even then, forget about creating a tornado or a full-blown earthquake that destroys the whole town, as the range of a spell tends to be limited to just a few meters. For spells that work if you're distant/not physically present on the spot, one can use sigils and/or enchant objects directly to make them release a certain spell when touched; that's especially popular for protection spells and such. Potions and crystals are also a thing that can be used to make certain spells more powerful and/or last longer.
As for magical effects... well... technically speaking, I would have loved for my magic to be "invisible", aka you're not really supposed to hear/see anything until the spell is released, but you're a mage yourself you can feel the energy "changing" around you. However, since I realize that doing something similar in comic form may not work very well, I decided to go for more traditional "magical" effects, sometimes coupled with faint sounds:
In this particular case, since we're dealing with a shapeshifter, I also chose to represent the change of voice when transitioning from one form to the other by using a gradient on the balloon outline