Add a rainbow with gradient tool(Russell's Rainbow), adjust blending mode and add some blur. Also add mist round the mountains.
step 4 of 7
Cut the girl in source 3, place on the grass and transform to be real size. Also add some shadows on the grass.
step 5 of 7
Add some gradient maps to warm the photo and adjust colors. If you like, you can also adjust levels, too.
step 6 of 7
To be more fantastic, add some shades(black, multiply) and highlights(white, screen) in the areas that you want to be.(I added the four side of the photo and some inner place.)
step 7 of 7
Finalize the photo.(You can use high pass function to be sharper.)