There are as many ways to write magic as there are stars in the sky. However, I have a few handy tips to help out in creating the magic of your world. These also apply to superpowers, if that is more your bent!
.
.
1. Magic should not come for free. Whether it is a physical toll taken on the body, prerequisite years of study of ancient languages to be able to perform it, or the sacrifice of components, you are making extraodinary things happen. Setting prices on those things makes them more real.
2. Magic should have limits. Those could be the laws of physics that govern the universe or you can throw those out the window. If you do, however, be prepared to set some caps. This ties into point one, but essentially, avoid letting magic solve every problem or be an easy fallback.
3. Magic can be rare or ubiquitous or somewhere in between, but it should be alive. Even if your world is not earth, people, even if they're elves or dwarves, they will always be innovating and changing how they use tools. There's something to say for the mysticism of "the old ways" but an immediate way of adding depth to your chosen magic is a tweak here, an outlawed practice there. If your story is set on modern day earth, explain how people use their phones and the internet to look up and trade spells, for example. .
.
Hopefully that sparks some imagination in your creative process and helps you solidify the magical system for your world. .
.
Thanks for reading and please let me know what you think!