Filling in the blanks...

Designing and adding content to the empty rooms of the Act 1 map. 

My workflow:

Concept

• Come up with a functional idea for the room
• Design furniture and objects appropriate for the room

Build

• Create 3D models of the objects and place them in the game scene (where this screenshot is currently at)
• Arrange the layout of the objects for visual clarity and navigation flow
• Add gameplay triggers and playtest for flow

Beautify

• UV unwrap, add textures, lighting and effects 
• Optional: Add gimmicks, Easter eggs

It's an iterative process so I would jump back to an earlier step as the situation requires.