So! I have a few threads in the Cardcaptor Sakura universe and by and large we use canon worldbuilding but also by and large it's incomplete or just plain bad, so we've been supplementing and modifying bits of it as needed. Except we've reached the point where non-card magic is being explored, so we had to come up with something that fit the theme. This is that thing.
Sorcery
The magic system is called sorcery in universe, and people who practise it are called sorcerers, naturally. It's something you're born with, and genetic in a weird Harry Potterish way, with magical parents producing magical children but sometimes magical children being born of nonmagical parents. There are sorcerer clans, specialising in different kinds of magic and passing their knowledge on.
Mana
Sorcerers have a finite resource that's consumed when they perform magic which I'm gonna call mana. You start out with a certain amount of it and it doesn't change if you don't touch it. However, whenever you spend it to do magic, you recharge a little bit more than you used to have. So, for instance, if you had (say) 50 mana and spent (say) 5 on a spell, you'd recharge to (say) 55 instead of 50.
While you're actively doing magic it doesn't recharge, but doing anything other than that does. Variety of magical things makes your mana cap grow faster; each time you do a spell, your recharge pushes your mana cap a bit less than before - so, for example, if you do the spell above a second time you'll recharge to (say) 59 instead of 60.
If you try to do magic that costs more mana than you have it works but draws on environment magic and has unpredictable consequences, which get progressively worse the more negative your mana reserves go. The limit is not merely the death of the user, it can get quite catastrophic and destructive in addition to killing the user. You get useful magical feedback before actually performing the killing magic, it's not all trial and fatal error, you'll always know what you're about to do costs too much if it does. That's the only feedback you receive, though - if the magic you're about to perform won't send you into negatives you get nothing.
Mana is tied to "stamina," too: when you're low on it you get tired or sick more easily, need more food and more resting time, your mind wanders more, etc.
Affinity
People's magic has affinities to different kinds of concepts (explained below). In-thread, it has been shown that certain characters can have sun magic or moon magic, and this is a broad and rough version of this affinity. Doing magic you have more affinity for costs less and recharges more mana. Affinity isn't necessarily the obvious thing - Sadde has a broad sun affinity but also a strong affinity for shape changing magic which is moon magic.
Meditation and Senses
You can suss this sort of information out by meditating. Having magical artefacts you control around helps with this meditation, as does practice and having more mana. There are also magical senses that can be developed that way, allowing you to detect and understand magic (or at least its broad strokes) without meditating.
Sorcerers can also see certain magical effects nonmagical people can't, like the embodiment of certain Cards, magical auras, and ghosts. It's possible to use magic for fortune telling, and magical artefacts help with this (the Cards, for instance, can be spread tarot style).
Concepts
Most magic needs to call upon "concepts." They're tied together more or less like alethian witch goddesses are. There is a hierarchy of them, with the sun and the moon at the top and various other concepts at the bottom. You can call upon parent concepts to perform magic with subconcepts, which makes magic more powerful but more mana-intensive and can sometimes include other unwanted effects.
Will
The most basic expression of magical aptitude is merely the application of one's will on reality. This is typically not very costly, effective, or useful, and tends to not even be fully under the sorcerer's control.
Spells
Next up are spells, which involve actually calling upon the concepts. All one needs to do is declare intent out loud, call upon a concept, and spend mana. The longer and more complex the declaration, the more control over and power behind the magic you can have. An example of a spell would be something like "I call upon the sun, the soul of fire, the thunder god, smite my enemy and turn them to ashes, direct lighting for me!" The exact phrasing doesn't matter, as long as the idea and the intent are there. It has to be on a language you can actually speak, however, just repeating some sentence you read online won't work. If you're only partially fluent in the language you're using your spells will be proportionally weaker. Additionally, just making a chant very long isn't enough, because it has to be loosely nonredundant. You can call upon various different and loosely connected concepts, or ask them for different things, or detail your request arbitrarily, but just repeating the same sentence several times doth not a good spell make.
But even the most elaborately chanted spell has limits to how much power you can channel. It's typical to "prepare" a spell in advance by creating a focus for it. A focus is nothing more than an object engraved with "part of a spell." Since material and method of engraving don't actually matter, most foci are just paper with something written on them (think Japanese "paper seals" à la Naruto). To create a focus, you mark it with part of a spell, either symbols or words or phrases, and dump mana into it by an act of will. Then, when you want to cast a spell, you can use the focus to reduce how much mana you spend at the time of casting and create much more powerful effects. Once a focus is used for a spell, it's consumed and destroyed.
There's a tradeoff between generality and usefulness. You can create a focus with the word "sun" or a drawing of the sun on it, and it can be used to aid in the casting of any sun spells, but it'll be less useful and powerful at helping you cast a thunder spell than a focus with the word "thunder" on it would be. You can charge a focus with mana indefinitely, and that'll make it more powerful for spells it's used for, but there are diminishing returns there, too. Foci, while they haven't been used, can have magical properties in their own right, albeit less powerful than artefacts.
Rituals and Magical Artefacts
You can imbue objects with persistent magical properties or create magical artefacts and constructs with a ritual. To set one up, you need to create several foci, including one that's to be the "ritual circle": a circle with several symbols on it, large enough to contain all the objects and people involved in the ritual. A chant describing all the desired properties of the created or enchanted thing must be composed, calling upon all involved concepts at all hierarchy levels. Furthermore, aspects like geographical location, time of year, and time of day are also used in a ritual. A ritual can fail if it doesn't contain a sufficient number of references to the various concepts it's calling upon, or if its chant is not good enough, or vague stuff like that. When a ritual fails, it consumes the foci and mana and doesn't actually produce the desired effect.
Furthermore, a ritual consumes the mana it does "permanently." That is, after you perform a ritual, you don't recharge, whatever the amount of mana you have left at that point is your new cap, and you'll have to start building up from there. Consequently, if you ritual yourself to zero mana you lose your magic (and also any new children you have will most likely be magicless). More than one person can help perform a ritual. Any steps can be performed by any sorcerer, and as long as you've performed at least one step of a ritual you'll sacrifice an equal portion of your magic to everyone else who participated.
Examples of things created by rituals are the Clow Cards, the Clow Book, the Key of the Seal, and Kero. It's possible to create a ritual that would give someone mana and turn them into a sorcerer but the conversion rate is abysmal (at least 100:1) and this ritual has not been discovered yet.
The Clow Cards
Clow Reed was a very powerful English sorcerer who created a very large ritual involving everything known to sorcery to unify it into the Clow Cards. Those cards are magic made solid, and they don't behave quite like regular sorcery. For one, using them doesn't spend any mana, it "occupies" it. That is, if you have (say) 50 mana and you use a certain card, while it's active you effectively have (say) 40 mana, but as soon as you dispel it you return to your original 50. If you try to use a card, you can only get it to act in ways that "fit" inside your mana cap - for example, the Time card is one of the most mana-occupying cards, and stopping time for even as little as a minute requires a huge cap.
Since using cards counts as performing magic, however, your mana cap does grow by doing that. It does so more slowly than performing actual spells, but it's not nothing. Capturing a card gives you a significant boost to mana cap (relatively speaking), as does figuring out new ways to use cards. Accepting a contract with the Key of the Seal ties your mana reserves to it and makes it both easier to perform magic with the staff and harder to do it without it. It's possible to break this contract, at the cost of losing the instinctive connection to the cards you get and the boost to your mana that owning the key/staff gives you.