Creating the magic system for you world can be a lot of work. It also takes a lot of time. Personally, I have never created a full magic system for any of my stories that require one. However, I have understood the rules of magic.
What can magic do?
This is an important question I like to answer. Can magic raise the dead? If so, why aren’t zombies roaming around? Is this type of necromancy magic forbidden? What is the penalty of being caught performing such magic? Have a lot of magic users tried?
These question bring to mind a certain character where his family or someone close to him/her has died. if such magic existed, would they go out and find a way to raise the dead or find someone to do it for them? That simple fact – whether or not magic can raise the dead – cane create an entire story for that character. I also think about the popular manga, Fullmetal Alchemist, by Hiromu Arakawa. After failed attempt to resurrect their mother by using alchemy, the boys lose body parts and go on a journey to find a way to restore their bodies.
Can magic turn copper into gold? Or can magic users turn anything in general into gold?
It’s the transmutation question. This questions brings to mind a thief/peddler who is not really good at magic but he does have a small gift to transmute certain things to appear like gold. With the system I created, this is possible. However, I put in place limitations.
The transmuted object will always return to its original form. The effect is not permanent otherwise the economy wouldn’t exactly work.
What does magic require?
For example, if someone tries casting a spell, do they need certain ingredients? Can those ingredients be replaced? Do they need candles? Or perhaps a magic circle? Does it need the person’s willpower? What fuels magic? Does it burn away people’s life-force? Does it need “mana” or other forms of energy?
With this, remaining consistent is key. Otherwise readers might feel like there’s nothing concrete. There’s only so far suspense of disbelief will stretch.
Any limitations on magic?
This questions goes back to the beginning. It’s important to know what magic can’t do. There are times when I need to start backwards. I need to place limitations on what magic can do because otherwise there might be plot holes. Why couldn’t a character do this spell at the end when he could do it at the beginning without any problem?
The rules are up to you to decide. Personally, I like to keep things consistent. It also helps to keep a notebook with all the rules for magic. If I ever need to go back and double check something, I have a place to go for easy access.
Finally, beta readers are a good resource to use. Those who read your story will, hopefully, point out things that don’t make sense.