Make a new layer, name it floor.  Flood fill this with white.  Move the floor layer underneath the back wall and left wall layers.  Click on the Draw tool and use these settings.

Pick a bright color and place the cursor in the lower left corner.  Draw a line up to where the two walls meet.  Now draw a straight line along the bottom of the back wall.  Move the floor to the top layer and zoom in to use the Paint brush to make any edits to the two lines.  Make sure they meet correctly and that in the lower corner it extends all the way down.  Now move your floor back under the two walls.  Don't worry about the blue line showing.  Later we can either delete it or use it to make a baseboard.

Before you go to the next step, right click and save the image below, greenmarble.jpg.

Making sure the floor layer is active, use the magic wand to select the area inside your lines.  Open the greenmarble.jpg.  Go to the flood tool.  Change the fill style to Pattern.  Choose your green marble as the pattern.  Flood fill the selected floor area.  Note here:  This method will work with the right pattern, but may not work with some others.  Marbles, cloth, textures, etc. seem to work the best.

Now what about that blue line?  Use the magic wand to select it.  Go to Selections, Modify, Expand and expand by 3 pixels.  While the line is still selected, air brush it with white.  Copy and paste this as a new layer and call it baseboard.

At this point it might be easier to view this layer only.  You can do several things with it.  For this example we will use the pattern fill and fill it with a wood pattern.

Right click to save this wood pattern.

Make all the layers visible again.

If you want, you can soften or blur the wood and decrease the opacity.  You could also use the Colorize to change it to a color that matches the walls better.