Sirius had been forced to learn it the hard way.
The Patronus charm was not a type of magic that was commonly found in the Black Library, in fact, you would be hard pressed to find a book that so much as mentioned in incantation. The Potter library had been of a little more use, but that still wasn't much.
Sirius truly didn't understand how Remus could study spells and wand movements from books. They were so vague! The explanations were impossible, and the wand movements never clearly described. Had none of them thought to put pictures that showed the entire movement? Really?
Remus had helped him with an exasperated sigh, after he had walked into the library with a huff and sat down opposite him.
"Expecto Patronum!"
A weak mist was spat out the end of Sirius's wand. It had been two and a half weeks already, and this was the only thing he could produce. He didn't know when Remus had started learning his Patronus charm, but Sirius was extremely jealous.
"Again, Padfoot. You're improving," Remus said approvingly.
"Don't lie to me!" Sirius whined. "I've been producing the same thing for the last three days."
"No, you've been producing it consistently over the last three days, and that's an improvement from your once in every thirty tries."
"Remus!"
It was two months later when Sirius's Patronus finally took on a corporeal form, and Sirius lost his concentration within seconds in excitement. It was a grim-like dog that looked just like Padfoot, or so Remus had told Sirius, looking as proud as Sirius felt.
"You did it Padfoot!"
"Now, it's time to knock James over in payback!"
Remus groaned.
Written for Ultimate Chocolate Frog Card Club: Andros the Invincible [include a patronus]