Add a new layer and call it bottom 1.  Use the selection tool to draw a rectangle that is wider than the fireplace about the size as you see below.  Since we are going to be using the deform tool on these pieces, if you make it bigger to start with you get a better look by being able to squeeze it to make it smaller.  Use the flood fill to fill this selection with the wood or fill of your choice.

Click on the Deform Tool while the bottom piece is still selected.  Put your cursor on one of the top corners and hold down the Ctrl key.  Slide the cursor inward to squeeze the top sides in.  Try to match up the top with the two sides so that they meet.  While the deform is still on, squeeze the wood piece up from the bottom by   letting  go of the Ctrl key and putting the cursor in the middle of the bottom.  Move the selection upward.

If you look at these two screen captures, they should give you a good idea of how your deforming should be coming along.

Your layer palette should look like this.  Move your bottom piece under the back wall.  Zoom in and use the deform tool again if you are slightly off in getting everything to match up.

Now to add the second section of the bottom.  Create a new layer and call it bottom 2.  Look at the capture below to help you figure out how big to make your selection for this piece.  Click on the Deform Tool and put the cursor on a top corner and hold down the Ctrl key.  Squeeze the top inward.  Now let go of the Ctrl key and put the cursor in the middle of the top and squeeze down.

Use Brightness and Contrast on bottom 2 with these settings once or twice to make a difference in the front bottom section from the back one.