The location of the Secret Door can be found in the beginning of the level, in the wall near the Ninji. The location of the Secret Door lies in the second area. Mario must use the star to become invincible and head to the wall near the Poison Bloopers. By using the Paperize abiity, Mario can place the door.
In this way, how do you open the secret door in Paper Mario?
Make your way into the mansion and head into the first door on your right. Then go through the door in front of you and head straight down the hallway. Go up the stairs and at the top you will see a outline of the secret blue door. This door actually is a portal back to Decalburg.