An example:
Although the subject is nice enough, the dull sky is spoiling the picture. But because it has a fairly even colour, it can be selected easily with the tool "color select" You can then reverse the selection , whereby you do not have the sky, but the geese and the dike selected. You copy this selection to the clipboard.
What you need now is a more interesting sky. Whenever I go out with my camera, and I see an intersting sky, I take photos, which I save in a file that I named "textures" . So now I go to this file and choose an appropriate sky:
With this new sky open in Photoshop, I give the commend "paste" and the geese with the dike will appear on top of the sky as a new layer. We now move the geese around on the new sky, once you are happy with the position, you merge the 2 layers to one layer, use the crop tool to cut out any excess sky, and this is your result:
A much improved picture, much more interesting to look at. Easy, is n't it?