When Hermione asks you to get the Marauding With Monsters book and get the spell book to save Neville, go to the second floor and sneak past the Prefects. Go into the library and get the book. Get down to the main entrance and get past those Prefects. Then, go outside. Stay away from Prefects and get to the Herbology Greenhouses near the forbidden forest and Hagrid's hut. Do not get caught by Professor Snape. Go into Greenhouse #3 and get the charm. Also, fill you potion vial with Wiggenweld. For a small shortcut, get caught by anyone but Snape. Go in and get to the second floor. Once there, you will see an intermission sequence of Nearly Headless Nick petrified.
On the wall there will be a message:
The Chamber of secrets has been open
Enemies of the heir... beware
Professor McGonagall will take you to Dumbledore's office on Floor 3. Go back to the seventh floor and use Diffindo on the tapestry. Do not go yet. Go in the room and there will be a Famous Witch's or Wizard Card inside.