It's tricky. Getting into and out of her rose-sphere is routine after a few days, but she still hasn't uncovered an explanation of how channeling works, though she's been informed that there are three options for how to do it (by herself, through a willing helper, or through an unwilling helper). Unlike the fourfold options for reaching the Dreamworld, this does not tell her how to begin fumbling towards a practical understanding, even though she can rule out the last one just on the basis of its description.
Beast can only do so a tiny bit of reading per day, and her pile of books that she can read shrinks much faster than his. She combs the library for multilingual dictionaries, so she can pick her way through the titles of the foreign-language books and at least prioritize them before handing them over.
Finally something in an obscure language that even Beast can barely read - Belle has to look up a lot of words - explains the channeling in a way she can understand.
To cast a spell, you need power and a way to form that power into an effect in the world. (Power can be drawn from many things; the text says 'from the Heart, from the Will, from the Sky, from the Earth, according to the needs of the spell and the resources of the Mage', and doesn't elaborate.)
If you work using yourself as a channel, you risk being distracted by the pain of channeling. The book recommends working this way for small spells, where the pain and the consequences of failure will also be small. It is also careful to specify that there are no permanent effects on the caster from working this way.
If you work using a willing channel, you will be free to cast your spell without the distraction of pain, but after a few uses your channel will start to suffer remnants of channel-pain even when you are not casting through them. (The Beast seems to think this is very funny.) These 'Echoes' can last for years, and get stronger the more power someone is made to channel, but after enough time spent resting they will fade away to no ultimate lingering effect.
If you work using an unwilling channel, every use will cause damage to the channel's mind, leading eventually to death. Even one spell, if it is a powerful enough spell, can burn out an unwilling channel to the point where they forget their own name. There is no known way to heal the damage caused by forcing power through an unwilling channel. The book strongly recommends against this method.
"...Accumulating lingering pain every time someone you're trying to help uses magic doesn't sound terrible? It sounds potentially much worse than being alone in a magic castle, to me."
"It does sound like it can get very bad indeed, though." This book has a handful of illustrations, including one of a man in rather wretched apparent discomfort with an enchantress standing over him. Belle translates the caption one word at a time: "To make... a fortress... suitable... for... an enchanter," she says as she picks her way across the phrase. "The Toolkit said something about counterspells always taking at least as much channeling as the original. I don't know if this castle is a 'fortress suitable for an enchanter' or not, but it's probably close to the same power requirements. I wonder who she channeled through."
"Maybe that was why she screamed so much. It looks like the channel needs to be there when the spell is cast, and there wasn't anyone else around."
"...This having been a long time ago, are you sure all you did was run into her and laugh?"
"My father was a very unpleasant person," he elaborates.
"My impression of her sanity only drops over time. Did she at least include him? Has he got his own hiding castle somewhere?"
"So," Belle says, turning to her notes, "my impression is that I'll be able to handle it for small spells - anything in the Hedge-witch book, probably, I can look for my father first once I know how to turn having decided on a channel into being able to channel a spell through same - but I don't think I'd be able to hold together any more significant concentration through the medium-sized magic, let alone the larger things that I might need to do to disenchant the castle."
"And all these books are very sure that it's bad to lose concentration on a bigger spell, though none of them will tell us how."
"There are plenty we haven't dug into yet," Belle points out, "but yes, it's something I'd like to avoid finding out the hard way."
"We keep looking," she sighs, "so we'll at least have the option of disenchantment, maybe, when I've learned enough. The effects of willing channeling through another person aren't permanent, and from what you've told me, failing at breaking this curse is."
She reaches for a new book.