A magic system becomes convincing when readers can understand its basic rules. You do not need to explain every detail, but the story should remain consistent about what magic can and cannot do.
Start with access. Who can use magic, and how do they learn? Then define limits. Are there abilities that remain impossible? Does magic require materials, training, energy or a particular condition?
Consider the cost. If a spell can solve any problem without consequence, it may remove tension. Costs can be physical, social, emotional or practical. They should matter to the characters and fit the world.
Think about the wider effects. If magic is common, how does it change work, travel, medicine, warfare or education? If it is rare, who controls it and why?
Finally, test the rules against your plot. When a character uses magic to solve a problem, make sure the solution follows the principles readers have already learned.
