Download the picture with girl and main ground and open it in Photoshop. The photo was taken with slightly different intention, that's why there is some orange clothing. You'll get rid off that in this step.
Create a new layer above the background. You can find the button "Create a new layer" in the bottom part of the palette Layers. If you don't see this palette simply press F7 on your keyboard or go to Window > Layers and it will appear.
Name the new layer e.g. REMOVE CLOTHING. It's good to name your layers because you can orient better than if you don't do it.
Grab the Brush Tool (B). Press Alt on your keyboard to activate the Eyedropper Tool and pick some light color from the sky. Now start painting over the clothing to make it disappear and let sky appear instead.
You can get the inspiration of where to paint on in the picture above. As I said paint over the clothing but don't paint over the straws! They create nice impression of space so you want to let them in the picture.
Then paint over the bush in the right bottom corner. It doesn't look photogenic and you'll add some better in following steps.
After this step your image should look like the following.
As you can see the sky and whole background is very uninteresting right now - it's just some light blue color and nothing more. To improve it you'll add mountains to the background and sky in following steps.
Before you'll start it's necessary to make selection of the girl and save it. To do that grab the Lasso Tool (L) and select the girl.
Be as careful as possible while making the selection. If you don't do it carefully your manips will never look realistic. It can be boring and time consuming but it's totally worth it.
If you're not sure how to use the Lasso Tool (L) or want to know about other methods of blending you can take a look at the article "A Photomanipulator's Guide to Blending" here on Photoshop Tutorials.
As I said you need to save the selection to be able to use it later. To do that go to Select > Save Selection ... and as Name write e.g. Woman. Press OK.
Press Ctrl + D on your keyboard to deselect it. You'll came back to the selection soon.
Download the picture of mountains and drag it to the picture. Press Ctrl + T on your keyboard and resize the picture on proper size. Then press Enter to apply the resizing. Rename the layer on MOUNTAINS and place it on the top of all layers.
To blend the layer with the rest of the picture change its Blending Mode on Overlay. You can find the Blending Mode option in the upper part of the palette Layers.
Your manipulation should look like the following picture. As you can see the mountains are well blended with the sky but they're also on the woman which looks really weird.
Now it comes time to use the selection you've created and saved in previous step.
Add new layer mask to MOUNTAINS. You can find the button "Add layer mask" in the bottom part of palette Layers (same as the buton "Create a new layer").
Go to Selection > Load Selection ... and select Channel Woman. Press OK.
Grab the Brush Tool (B) and pick black color. Make sure that the layer mask is active (just click on it) and paint with black inside the selection. It causes that the mounatin on woman dissapear.
You should get something like on the picture below.
Press Ctrl + D to deselect the mask.
Mountains would look better if they're darker and less saturated. You'll fix it in this step.
First let's make them darker. To do that add new adjustment layer Levels above MOUNTAIN. You can find the button "Add new fill or adjustment layer" in the bottom part of the palette Layers similarly as the button "Add new layer mask" you've used in the previous steps.
Using of adjustment layers is much better than direct adjustments of the picture because this way you can always go back and edit whatever you need and you're not limited with the number of step which you have set to History remembers. Also you don't need to re-do anything. But back to the image...
Add new adjustment layer Levels and set the Input Levels on 37; 0,66; 255.
You want to the adjustment affects only MOUNTAINS and not the whole picture. To arrange that right click on the Levels layer and choose the option Create Clipping Mask.
To desaturate the mountains add new adjustment layer Hue/Saturation on the top of all layers and set the value Saturation on -100. This way mountains get the same color as the sky.
Create clipping mask from this layer as well.
After this step your image should be similar to the following one.
As I mentioned before the sky looks quite boring because of very few details. Let's add some drama!
Download the picture of clouds into your manipulation. Press Ctrl + T, resize it on proper size and press Enter. Name the layer CLOUDS 1. To blend the layer change its Blending Mode of Soft Light and place it on the left side of the picture.
On the following image you can see where to place the clouds.
You don't want to have clouds on the girl and snow in the foreground and you also surely want to have nice soft transition between clouds and the rest of the picture.
To fix all this add new adjustment layer to clouds. Grab the Brush Tool (B) and pick black color. Now paint over the areas you want to hide. You can get the inspiration where to paint on the picture below. Parts which could be hidden are red.
Right click on the layer CLOUDS 1 and select the option Duplicate Layer... . Place this new layer on the right side of the picture and name it e.g. CLOUDS 2. Change its Blending Mode on Soft Light and remove the current layer mask. Then add a new one.
To blend the layer CLOUDS 2 properly paint with black on the layer mask over the red parts from the following picture.
After this step your image should look like the picture below.