Statistically, magic in Charmed and Buffy are pretty much the same. There are no rules adjustments that need to be made at all. Once you know a spell you can cast it as often as you like as long as you can beat its power level with your casting roll. However, the styles of the two are very different. Firstly, as mentioned before, while anyone can cast a spell in Buffy, only a Witch can use magic in Charmed. There is nothing to stop someone gaining a purely academic knowledge of magic in a Charmed game, but they can never cast an actual spell. So there is far less written about magic in the Charmed world, and far fewer ‘magical tomes’. Charmed magic is also very ritualistic, and more cookery oriented. Many spells requires ingredients and components that often need boiling together to the right consistency. However, there are plenty of low-level spells that only require wrapping some personal possession in the right leaf.
The main difference in magic styles is the need to recite a rhyming couplet. This may sound rather easy, but not any rhyme will do. Like any magic words, the rhyme focuses the will and magic of the caster into the spell. So the words need to be right. Much of the research into a new spell will be finding the right rhyme to summon the energy properly. This is the main reason that only a Witch can use magic. Every Witch has a sense for what words will work; there is no science to it, only intuition. The Witch must try working out the rhyme until it ‘feels right’, much like any poet does. Once the words are right, she needs to find the correct ingredients and ritual, if such things are required. This is an academic exercise, requiring the Witch to hit the books and look up the right herbs and actions for the effect she needs. If the spell works, then the Witch can write it up in her own Book of Shadows so she can remember it for later. Casting the spell again doesn’t require her Book of Shadows; she only needs to remember how to cast the spell. So the casting roll for any spell also represents the Witch’s attempt to remember the right things to do. If the Witch has not had access to their Book of Shadows, or not been revising her magic, the Director may apply an increasing penalty to her casting rolls until she does some studying again. While their Book of Shadows isn’t magical in itself, old books with that much magical lore can acquire a certain enchantment. They also take on some of the spirit of the Witch, as they are such a personal thing. A group of Witches may all write their notes into one book, or keep their own individual notes.
There are less direct power spells in Charmed. Rarely does any spell allow you to cast flaming bolt of energy. A Witch’s special power allows her to use direct magic, so Charmed sorcery is a little more subtle. Most spells are used to destroy or vanquish a particular demon or entity. This means a Charmed Witch often needs to invent more spells than a Buffy one. Each time the Charmed Witch meets a new adversary she needs a new spell to defeat it. So it is not very often that she is able to use any spells she has already crafted and researched. If a Witch wanted to use a spell on an enemy, she often needs to craft it into a potion. Such a potion is made before she expects to deal with the bad guy, and then thrown at them when they are confronted. Often the potion just primes the target for the spell, and a verse still needs to be recited to cast the spell. This may be a little unwieldy, but does allow the Charmed Witch to blast her enemies with magical power. The Director should call for a casting roll when the potion is made, but not tell the Witch if it is a success. The Witch shouldn’t know if her potion is right until she uses it. However, if they rolled high, they will know that there is a good chance it will work. Another casting roll may be required when the potion is used, but if the potion is a failure the spell won’t work.
Magic in Charmed has less of an occult feel, and works more like asking a Genie for wishes. It tends to give you precisely what you asked for, so you need to be careful with your question. Just as in Buffy, misfires can occur in Charmed, and with alarming frequency. They seem to happen more frequently in Charmed, but are not quite so dangerous. You may wish to add a few side effects to any spell that only just succeeds, although the spell should still work. You may also wish to make things less dangerous for Charmed misfires. Charmed magic seems less of a powerful dangerous force, and more a strict mother. Misfires often teach the Witch in question a lesson about the flippant use of power, much in the way a Genie may pervert the spirit of a wish. For instance, when Pru wants to understand men, she gets turned into one. When Phoebe gets a flying power, she is unable to land. Magic is not an exact science, just like in Buffy. However, spell misfires tend to teach the caster a lesson rather than harm them or cause real danger. The danger comes from fighting evil when you are not at you best.