2. Flood fill this layer with a pattern or solid color of your choice, these will became the floor.
 

3) Click on the deformation tool.

To get the perspective you want, drag the top middle handle down to the height (roughly) where you want your flooring to go to in your image, then hold down the Ctrl key and drag one of the top corner handles inwards to get your distance perspective. (You will see that the cursor change ).

4. Add a new layer under  the floor for the walls,  again  flood fill with a color or texture.
 

5. Duplicate the wall layer and select Image/Mirror, you have to take care for to place the wall with the floor.
 
 
 

6. Add a new layer under  the the wall, and rename "front wall", apply again deformation tool, until these wall meets the floor and the others walls.
 

TIP: If your pattern not tile, apply the "clone brush tool" for to adjust.
 

7. Again create a new layer for the ceiling, flood fill with the color of your choice, and apply deformation tool, making sure the corners match perfectly.
 

8. If you agree with your room, select Layers/Merge/Merge Visible, and begin to decorate the room, for each objet add a new layer. When you are ready, select to desirable size and Image/Crop to selection.
For each objet you can apply drop shadow, and adjust brightness/contrast.

Thanks Tubes by Deanna