"I believe that's when he developed his obnoxious tendency to faint when he found himself outside the center of attention." But that was neither here nor there. Severus stood more beside Draco than in front of him, not exactly sure how to describe this bit of magic to someone like him.
"You know the functions of a Patronus, I expect. Its power comes from emotion - most people find a memory is the best way to focus it correctly. Something profoundly happy - something strong enough to channel into physical form. With the right amount of concentration, the proper direction, and the incantation -" He raised his wand, uttered the words, and sent his own Patronus leaping across the office and through the window onto the grounds - "It isn't so very difficult." Second nature, really, after a time.