How does one cast a spell?
When I was young I really believed it was mind over matter, and that we were all capable of performing magic if we only tried hard enough. Consequently I spent some time trying it out, and was disappointed. But I always believed that if I had enough time to really get down to it one of the methods I came across in novels would eventually work. And I really really wanted to be able to do magic. It's the coolest of abilities, isn't it? To talk to snakes, or fly, or make fire without having to rub little sticks together (because imaginary worlds in which magic is possible never seem to have got around to inventing matches).
But all of these constructs agree that magic is only a matter of application (as long as you have the germ of ability to begin with):
In David Edding's Belgariad series we have The Will and the Word. Will it to happen, and then speak the word to release the magic. Just remember that the laws of physics still apply, and don't ever will something out of existence, or you'll vanish yourself. Tricky.
Harry Potter needs a wand and a lot of knowledge to unleash his dormant power. Get the stress on the wrong syllable or wave the wand the wrong way, and anything could happen. Years are spent at school perfecting techniques, but in the end it all comes down to chance, too. Who knows why some wizards end up so powerful while others struggle to deal with a blast-ended skrewt? Is it genetics? Hm.
Middle Earth has very few wizards, and they are pretty much rock-hard from the word go. They were never men at all, so there was never a hint of having to learn anything; however, Gandalf does get a free upgrade after his battle with Balrog, which is nice. Here's a lovely Wikipedia page in which someone else with more time and energy than me explains exactly what wizards are.
Ged has to go to wizard school too, in A Wizard of Earthsea by Ursula Le Guin, but it's a bit different to Hogwarts. Ged has natural ability and soon masters magic, but unfortunately in the process he summons a pretty unspeakable nasty that stalks him through the world. Doh. Who knew that magic had a dark side? Um.
I'm sure you can think of many more examples of how a magician is made. I'm going to secretly continue to practice on the Will and the Word, as that makes the most sense to me... *wills chocolate hobnobs into existence*