Naturally this was/is an excellent question because reading about these wondrous & mystical realms is one thing, and writing one is a different matter all together. But often trying to craft your own unique system of magic can seem like the hardest part of all, so here – as I detailed to my friend – is how I go about it:
The first thing you must decide is how ornate and prevalent you want yours to be. Some, such as J.R.R. Tolkien, have a restrained use of magic: You do not see Gandalf casting a lot of spells or throwing fireballs in The Lord of the Rings; when danger comes, Gandalf draws Glamdring (his sword). Yet there is still a steady presence of magic in Middle-earth, subtle yet bright, clear, and elegant.Then there are authors like Rowling, whose Harry Potter is full of flashing spells - but with carefully laid and complex rules behind and them, making the magic system work and adding realism and depth to it.
In general, the larger the role a magic system plays the more complex it is. Neither style is better, just different, as while the restrained kind is typically easier to craft, the often complex rules of the ornate system can help drive the story. The restrained kind often lends to a more epic feel, yet the ornate can be analyzed by the reader and bent in truly fascinating ways.
After you choose, the rest – i.e. the actual system of magic you are striving to create – depends entirely on the world and characters you have crafted. For example, you will have to decide whether your system will conform/relate to one of The Nine Magics.
Inspirational readings I can also offer: The first five being some of the best myth-based Fantasies around, and the bottom two being the best examples or the more ornate yet unique/excellent systems of magic (beware, the two categories oft blur):
- The Sea of Trolls trilogy by Nancy Farmer (Celtic and Norse myths)
- The Secrets of the Immortal Nicholas Flamel series by Michael Scott (All myths, believe it or not)
- The Dark is Rising Sequence by Susan Copper (Arthurian legends)
- The Children of the Lamp series by P.B. Kerr (Arabian myths)
- In the Forests of Serre by Patricia A. McKillip (Slavic myths)
- The Abhorsen Series by Garth Nix
- The Vows and Honor trilogy and its companion By the Sword by Mercedes Lackey
Beyond this there is little more I can offer save one final recommendation: read the Sanderson’s First Law article, written by the acknowledged Master of Magic Systems, acclaimed Fantasy author Brandon Sanderson.