1) Change the name of the Background layer to "Layer 0".
2) Use the rectangular marquee tool to select the part of the building you'll want to use for the reflection. The vertical part of the selection should be large enough to allow for maneuvering into place later on.
3) Hit Cmd+J (Ctrl+J for PC ?) to put the selected area on it's own layer (layer 1).
4) Move layer 1 below layer 0. On layer 0 either erase or use a layer mask to remove the unwanted fountain area and allow the image on layer 1 to show through. Carefully maneuver the image into place (use free transform if necessary, but I didn't need to do that).
5) Now, use gaussian blur to slightly blur layer 1. You may also want to darken it just a tad. The object is to blend it into the image so it looks natural. If you erased with a soft brush setting, that should help the blending process.
6) That's it!
