Know where your magic comes from. Does magic have its source in life energy, the elements, deities, or other dimensions? Maybe something else. The source of magic can say a lot about what is important about your world because the foundation of magic is one of the foundations of your world – and maybe the only one that will ever be articulated. Also, whatever the source of your magic, it is something that has a direct impact on your civilizations. In many cases, the ability to use magic dates from the beginnings of the human (or nonhuman) race and the source of magic dates from the beginning of the world or universe. If you do not yet have a creation myth, thinking about these issues may help you create one.
Know the limits of your magic source. Is there a finite amount or
does it replenish itself? If it is replenishing, at what rate does it
renew itself? Does it need any help to renew? While a limitless pool
of power can be a heady possibility, consider placing limits to
foster possibilities for conflicts, dilemmas, and world-wide crises.
Nothing spawns conflict like competition for limited resources. And
very few crises top the possible disappearance of magic from a world
that depends on its existence or use.
Know the manifestations of magic. Is it mobile or static? Magic that
stays in one place could pose a challenge for magic users. Does it
form ley lines? Ley lines are lines of power that criss-cross the
landscape, sometimes intersecting to form pools of power called
nodes. Does it manifest physically? Can it be gathered or
contained? Crystallized magic, nodes, or some other form of
concentrated magic would be a resource for magic users and anything
that needed magic to survive. Is it spread evenly across the land or
are there areas of higher concentration? Maybe it gathers around
people or places or certain types of events (wars, strong heart
wishes, pivotal moments in history, magic users).