You have to gather mana before you can cast a spell. This takes an action (if you critically succeed, beating the gathering difficulty by 10+, you can do it without blowing an action).
The difficulty to gather that much mana is equal to the mana cost of the spell you want to cast.
There's no exhaustion effect for casting spells... it's slower than normal actions (one action to gather, one to cast), but a mage can cast all day provided that there's sufficient mana.
If you're near a mana font, the gathering roll is easier. If you're in a dead zone, it may be impossible or at least much harder.
Every mage gets the Mana Gathering skill. It's based off of... Knowledge? Drive? Hm.
Sustained spells (much like Delayed ones in Ironclaw) reduce your Drive as long as they're in action. If reduced to zero or less, you can't sustain any other spells... it's just too draining.
It takes an Edge to get access to Mana Gathering and the spell-casting skills. Each Edge will grant you access to a small set of spell-casting skills appropriate for the kind of magic you're studying.
A font of mana has a Mana rating. This adds to all mana gathering rolls in its vicinity. It also helps determine the difficulty of absorbing it and achieving godhood (probably something like Mana x 5 + 20). Oh, and if you get too close to it, you'll take its Mana rating in damage per round until you extract yourself.
If you have a Mana rating yourself (only gods do), you act as a font of mana with that strength for everyone in your vicinity. It adds double to your own mana gathering rolls.
For every 5 points that you exceed the necessary mana gathering difficulty by, you get a +1 to the casting roll.
Absorbing mana is generally very obvious and draws mystical blue sparkles into you from all directions. If you want to do it slowly enough that it's not visible to the naked eye, roll Mana Gathering at -10.