After cloning I found them a bit blurred. To correct it, go to Filter-Sharpen-Unsharp Mask and fill the following values:
Use Curves to darken the hill:
As you see on the final result preview, the main light comes from top center . So on Curves layer mask I erased the higher part of the hill that's nearer the sky (to keep it brighter) and keep the lower part darker (don't erase or just erase with lower brush opacity ).
Apply Photo Filter to change color of the hill a little:
Open sky 1 stock. Choose sky part and place it on the top (the black part), set this layer above the background and under underground group. I rotated it and tweaked it a little:
Use layer mask to remove unnecessary edges:
To make sky look smoother, go to Filter-Noise-Dust & Scratches:
I named this layer "main part".
Copy a part from sky stock and put it at the left side of the main part, resize to be smaller:
Remove hard edges of this part with layer mask:
I called this layer "part 2".
Add another part from sky stock to our picture. I placed it at the top right, used layer mask, applied Dust & Scratches filter with same settings in step 38:
Continue adding another sky part. Move it to the right corner of the sky, apply Dust & Scratches filter and layer mask:
Make group for these sky parts, name it "sky 1" or whatever you want. Use Curves to change color of sky 1:
Apply erasing job on Curves layer mask:
Make a Color Balance layer within this group:
Erase on Color Balance layer mask again with brush with opacity about 45%:
To add more details to the sky, I used sky 2 stock. Place it over sky 1 and change the mode to Soft Light 100%:
Name this layer "sky 2".
Use layer mask to reduce some details of the sky 2:
Apply Dust & Scratches filter with radius 2 px:
I used a Color Balance layer for sky 2:
Continue with a Selective Color layer:
Darken the sky a little with Curves:
I used a Levels layer to darken the sky 2 again. Then I erased the top center of the sky to keep it brighter than the sides:
Make a new layer on the top of layers. Use soft white brush with 50% opacity to paint on the top center of the sky. Change the mode of this layer to Soft Light 100%:
New layer and use soft brush with color #f7d17f to paint on the top center of sky again. Lower opacity of this layer to 30%:
Open tree stock. Take its trunk and put it on the top of the hill after scaling:
Use layer mask to blend the tree with the grass:
To make shadow for the tree on the hill, I created a layer under the tree one. Hold Cmd/Ctrl while clicking tree thumbnail layer to load its selection:
Fill this selection with black (Shift+F5). Then choose Edit-Transform-Flip Vertical and move this shadow under the tree bottom.
Lower opacity to 60% then apply Gaussian Blur with 3 px (Filter-Blur-Gaussian Blur):