I'm not a hard-core fantasy writer, although I do have some magical elements in my last two manuscripts. Everyone at WIFYR, however, told me that I had to go to Holly Black's class whether I wrote fantasy or not. I'm glad I did. She puts a lot of thought and effort into her books, and I really enjoyed thinking about creating a magical world as she set it up for us.
There are two types of magical worlds: 1) Open, where everyone knows magic exists, and 2) Closed world: where very few know
1. Who has it? Small number, magic requires sacrifice or training
2. What does it do? Limit scope, protective, defensive
3. How do you make it happen? Action, ceremony, e.g. bare hands touching bare skin effects result. Make it complicated, lengthy ceremony, difficult somehow
4. How is user affected? Drain on energy, premature aging, must obey magical organization's rules, pain, madness
5. How is world affected? World in danger, magic causes blights, disrupts machines.
6. How are magic users grouped or perceived? Users of magic thought to be immoral/evil/inhuman—taking it up means limiting self in society.
All magic has to have a price
When Kathleen Dewey wrote Skin Hunger – price of magic school is that only student lives, rest of them die from the lessons.
What are the limits of magic? It needs to be calibrated. Can’t give them so much magic that they can do everything and get out of every situation. Limit effectiveness.
Laying cards on table about the mystery: they need to see the map [how it works] before they have to use it. Better to be clear than fancy. the best thing is to let the reader know. It is the character’s need, how high the stakes are, rather than the not-knowing, that will keep them reading. A character with deep need and higher stakes will buy the author a little more time to reveal things.