The video linked previously explains the process fairly well. I do depart a little since I have a slightly different way of working, but the basics are there.
I first create a rectangular selection with the rectangle tool
. I manipulate the rectangle into a perspective I feel works best for the given image. in this case I used the water as the common horizontal plane, keeping in mind that the lighthouse, rocks and jetty all rise above that plane and the cutaway view of the strata lies below it.
Once I have the perspective I add the lighthouse and rocks to the selection anywhere it breaks out of the rectangular selection area (mostly at top) and remove from the selection area anything I want to show in the stratum (the foreground rise of the road leading to the lighthouse). I then create a mask.
The rest is sourcing images to represent the water and rock below the cutaway and putting it into perspective of the 3D tile. I mask it into a rectangle in perspective and here is where you can determine how thick you want the tile
to be. In this case I went with fairly thin. Some understanding of geology and physics is used to make the cutaway look plausible. The final touch was adding the birds circling the lighthouse. Just another element to add the illusion of a game piece or mantlepiece figurine.