I approach "magic," like most objects of creative design, as a system. The form and function of a system of magic emerge from the nature of its internal elements and how they are connected.
If you just like designing systems, hey presto! You're done. :) But if your goal is that a system of magic should fit into and explain and enhance its larger setting (a story, a tabletop RPG, a computer game), then designing a system of magic needs to be a conscious process of directed design. That means choosing the right systemic elements and organizing them so that the form and function of magic that emerges from those choices meets your larger creative needs.
I might have more to say about that process. :) For now: hello!