i start out with a rough idea of what i want to make. usually in my head cause i can't draw.
find the stock images you want
cut them out (pen tool is the easiest)
stick them roughly where you want them to go
resize the layers to fit
group your layers so you can find them later as you add more layers and adj. layers
edit individual layers for color, light and toning for individual pieces (clipping masks make this easy)
cuss, redo the entire thing
add some global color, tone and texture (textures hide a lot of sloppiness)
flatten and save
Great A'Tuin is the Giant Star Turtle who travels through the Discworld universe's space, carrying four giant elephants who in turn carry the Discworld.
No-one knows why A'Tuin flies through space, but one theory is that A'Tuin is crawling from the Birthplace to the Time of Mating, as are all the universes worlds, obviously also carried by giant turtles. When they arrive they would briefly and passionately mate, for the first and only time, and from that fiery union new turtles would be born to carry a new pattern of worlds. This is known as the Big Bang hypothesis...
Nip down to your nearest Waterstones, give the nice man behind the counter some pennies and purchase a copy of The Colour of Magic by Terry Pratchett, and be prepared to spend all night reading and laughing until your sides hurt...