Harry and Ron learn that Ginny has been taken into the Chamber and go to offer their knowledge to Professor Lockhart. When they discover he’s trying to take off (and that the things he claimed to have done in his books were actually done by other people), they instead disarm him and force him to join them. They go to Moaning Myrtle’s bathroom. After asking Myrtle about how she died, Harry is able to figure out which sink covers the entrance to the Chamber of Secrets. Focusing on a little carved snake, Harry uses Parseltongue to open up the huge pipe down to the Chamber. He slides down, followed by Ron – who had pushed Lockhart down the pipe first. Once at the bottom, Lockhart physically overpowers Ron and grabs his wand. He then explains that he will take the basilisk’s shedded skin back up to the castle as (false) proof of his victory over it. He tries to use a Memory charm on the boys, but Ron’s damaged wand backfires, causing a blast that injures Lockhart and creates a wall of rocks separating him and Ron from Harry and preventing them from going any further.
The text says that Harry, Ron, and Lockhart went down some stairs as they went from Lockhart's office to Moaning Myrtle's bathroom. Moaning Myrtle's bathroom is on the second floor, (CS8). Lockhart's office is also on the second floor, (CS6). Lockhart and the boys shouldn't be going up or down any staircases at all.