Thinkin' About My Doorbell
The player has to open the doors in the correct order, 1, 2 and 3, as labelled in the image below. Note that the player does not necessarily have to open door A before B. For example the achievement will unlock if the player opens 1B then 1A, then 2B followed by 2A, and 3B followed by 3A, so long as the doors are still unlocked in the proper 1, 2, 3 order.
The achievement title is a reference to the White Stripes song My Doorbell.