Here's a quick guide to casting spells in my Crux Fate game. It borrows from all over, but mostly I'm using Mage as inspiration here. To cast a spell, you need to do this:
1. Declare you are going to try to cast a Spell.
2. Describe the spell effect you want to accomplish.
3. The GM determines if that is a Cantrip or a major spell. If it is a Cantrip, it activates and that's that. If its a major spell, you need to use Sorcery or Faith to determine the effect. You'll use that skill to perform one action, which will be the spell effect (attack, defend, overcome obstacle, discover, or create an advantage). Spells created as advantages have stress tracks, which can be attacked by attempts to dispel the spell (the base number of stress boxes is 2, plus any shifts you wish to spend on them).
4. You must pay 1 mental stress to cast the spell, unless it is a cantrip. If its supposed to be a permanent (or long enough to be the same thing) you must spend a fate point as part of the cost of casting a spell. Fate points can be avoided in the casting cost if the right material components are provided for casting the spell, most often this isn't necessary, though.
Minor Spells: "Cantrips"
Minor spell effects are effects that don't need any sort of mechanics to be turned on. They don't deal stress, they don't create aspects and more or less are window dressing effects. Nicknamed cantrips here, you don't have to turn them on for them to work. You need to have justification for them, like an aspect or stunt.
Example: Zhaul Wolfsfang has the class aspect Wolf Clan Urzahadi. Since Urzahadi are a unique Ursyklon version of Druids, he wants to use the aspect as justification for a cantrip to create some water. Since he just wants to create a tiny bit of water, like a gallon or so, its a spell the GM is fine with. He creates a gallon of water, using it wash off his hands.
This can use this to do a variety of minor effects, from create light to sending messages on the wind. Although this might seem somewhat potent on its own, it still is reserved as a restriction. Should a spell effect need any mechanics to work out, it should be a major spell. Alternatively, you can choose to let players of spellcasters to use Sorcery or Faith to deal direct attacks like it was Brawl or Shoot. Consider creating a stunt to allow that if you think that would make Sorcery or Faith too potent of skills.
Major spells require mechanics to work, a whole host of effects that might cause stress (attacking the enemy with a fireball) or might alter the situation drastically (creating an aspect like Control Weather). They might also grant you some form of justification or permission to do other things. Cure spells, for instance, allow you the chance to justify lowering the time some kinds of physical Consequences take to be reduced. Spells cannot ever cure Severe or Extreme consequences, those always take time to fully recover from.
Major spells always cost 1 mental stress.
The fast and dirty way to handle most major spell effects is to give them an aspect through the Create An Advantage action. Major spells have their flavor determined by what kinds of flavor you've chosen with the stunt that lets you spellcast. You are restricted to those one or two choices, which often can force you to be creative (how do you use Abjuration to boil water, for instance?)
If a magic effect is something permanent, or fiat, you need to spend a fate point to create it as well.
Permanent Enchantments & Magic Items
To create a magic item or to make something permanent, you need to spend a fate point. Typical items vary, but most work like this:
Potions, Elixirs, Scrolls: Anyone can use these, even nonspellcasters. Potions and elixirs require that you drink them; scrolls require a successful Sorcery or Faith check to cast the spell in the scroll- opposed to the Sorcery of whoever created the scroll.
These items provide boosts for a scene. Fiat effects from scrolls will require the user to spend a Fate point- but scrolls can't be turned into permanent effects.
Wands, Staves, Rods: These take the form of stunts most often. Not what you were thinking, right? Welp, these are object based stunts- meaning you get the stunt so long as you have the item tied to the object. This is also to allow some diversity in the kind of item. In PF, your Wand of Magic Missile will be just that- but maybe the one you've created also does a bit of abjuration, and you made it from oak. So maybe Oak Wand is a better name for the stunt:
Example Stunt: Oak Wand. So long as you are holding the Oak Wand, once per scene you can spend one of its stress instead of your mental stress to cast magic missile or an abjuration effect. The Oak Wand has two boxes of stress, and it recovers its stress only when you have time to properly re-energize it (i.e., at a major milestone or similar event).
That is one way to do it. You could also be very specific on the kind of spell it casts, in exchange for more stress for the item in question. Two stress boxes is ideal for this sort of thing I think.
Wondrous Items: Like Wands and Staves, these most often provide item-based stunts.
Magic Armors and Weapons: These should be named. Outside of that, they provide a new aspect and some new justifications. That's about that.
Note that magic items and permanent effects still require time and resources to create. If you don't have necessary Resources, you might have to take on consequences or other means to provide that if you don't have them. Time as well- some items can a day or days to be created.
This stunt lets you cast major spells. Casting them costs you 1 mental stress. You can only take one flavor of spellcasting: Alchemy. In addition, you can create elixirs or potions without having to pay fate points. Note that crafting these items still requires some time and resources to create, however.
This stunt lets you cast major spells. You have to select two flavors of magic, except one of these flavors has to be Exploits. You are used to breaking or bending the rules of magic, and can exploit them well. You can choose to spend a Fate point during a scene to change one of your chosen flavors of magic to one of the following instead: Fire, Water, Earth or Air. This lasts until the end of the session, when it reverts back to what it used to be.
You can use Sorcery in place Weaponry or Brawl for attacking. In addition, you can choose to cast a spell through the weapon you wield, charging it with energy (elemental of whatever kind that seems apt). You take 1 mental stress and you gain a +2 bonus on your next attack with it.
Ok. That's it for tonight... I feel better getting that all written out somewhere. Anyway, thanks for reading.