The Preperation:
Fred & George were glad to set up a decoy for Harry to sneak into Snape's office after class. They set off a display of fireworks in the corridor. Fortunately for Ron, Harry and Hermione, the 4th years had been making Polyjuice potions so Hermione just had to steal a little bit of one that looked well made. It was Ron and Harry's job to get a hair from Malfoy & Ron had to get a hair from Umbridge.
During Care of Magical Creatures the next day, Harry was exceptionally careful around Malfoy, (having recovered from his injuries fine). Harry did his best not to burst out laughing at the thought of what they were about to do.
Harry slowed down going to dinner he stayed behind a little. When Malfoy wasn't looking he silently did a tripping jinx, causing to Malfoy fall and his stuff to spill everywhere. "Petrificus Totalus!" Harry said. Malfoy's body was immobilized. He plucked a few hairs from Malfoy's head, laughed in his face and then said Iinvisibalus!" (Harry had learned the invisibility charm from D.A.) Malfoy was no where to be seen. Harry smirked.
Meanwhile Ron was busy getting a hair from Umbridge. Wearing the Invisibility cloak, he went to her office, surprised to find her still there. He snuck around the corner and quickly said. "Petrificus Totalus!" While she was immobilized he took out one of her hairs and ran to locked her in her office. He put a muffling charm on the door and ran to the Gryffindor common room.