Being a fantasy writer, one of the biggest parts of building a world often includes creating a magic system. Personally it’s also one of my favorite parts. Writing in itself is a kind of magic, but where my ability to make things happen is limited to what happens on the page, the limits are entirely set by the rules of your world.
Any magic system needs to hit three points: the rules, the limits and the costs. Magic in itself can very well take any form you please, be that the wand-waving spells in Harry Potter, to elemental prowess from Avatar and even into psionic powers such as in Matilda. Regardless of the form, those three points dictate how magic works.
Rules define what magic is. This includes the form it takes, what the power source is and who can have it. This also covers any laws or regulations you may have. Consider things like if there are uses of magic that might be illegal, or if magic is outlawed entirely, if there are ways of using it legally. When determining a power source also consider if there’s a way to measure how strong magic is, and what the difference in power levels might be. As you figure out who can and can’t have magic, also look at when magic most commonly expresses itself; after all you may not want a toddler with the ability to demolish buildings.
Limits are self-explanatory. What can magic do and what can’t it do? It also helps to know what happens when someone attempts to push passed the limits of what can be accomplished with magic.
Costs of magic splits between physical cost and material cost. Physical cost includes energy, willpower or even lifespan. Once you’ve determined what a magic user pays to physically activate their magic, consider if they can push the cost off onto someone or something else. This may cross over into material cost: what does your character need to direct it? This may include things like well-known spell ingredients eye of newt, the magic wand and books. For systems that require learning, this is another part of cost: what it takes to gain magic. Keep in mind as well that not every system has a material cost.
Addressing the rules, limits and costs of your magic system gives you a framework to build specific details from such as spells, rituals and magical items. It can also open up new questions as you flesh out the system, helping you solidify and diversify magic in your world.