Preview of Final Results
You can find list of all resource here.
Step 1 - Removing the Sky from the Background
Open the background picture in Photoshop. Right click on the layer and select the option Layer from background.
First thing you need to do with the picture is to remove the uninteresting sky to be able to add the more dramatic one. You can easily do that with Channels.
If you don't have the palette Channels visible go to Window > Channels and it will appear. On this palette you can see four different layers - RGB, Red, Green and Blue.
Right click on the Blue channel and select the option Duplicate Channel... Make visible only this duplicated Channel.
Go to Image >Adjustments >Levels or press Ctrl + L on your keyboard. Set the Input Levels on 128; 1,00; 232 and press OK. You should get something like on the following picture.
You want to make the picture only black and white without any shade of grey. Things you've done before leads to creating the mask for the sky. White color shows part of the picture which will be selected and black part stays unselected. If you let some grey colors in the picture they will be selected e.g. with 50% opacity.
To make the whole ground black grab the Brush Tool (B), pick black color and simply paint over the grey areas of the water.
On the horizon there are some buildings which shouldn't be in the final picture. Carefully paint over them with white brush. You should get something like on the picture below. On the following picture you can see how the channel should looks like.
Once you're finished make visible all channels except the one named "Blue Copy". Go to Select > Load Selection and set the window which appear as on the picture below. Press OK.
It activates the mask based on the Blue copy Channel.
The sky is selected and you want to hide it. First you need to invert the selection. Do that by pressing Ctrl + Shift + I on your keyboard or go to Select > Invert. Then click on the button Add layer mask which can be found in the bottom part of Layers palette.
Thanks to it the sky disappears and you're able to add another, more dramatic. Below you can see how the picture looks without the sky.
Step 2 - Removing the Cottage
There are fragments of the cottage in the left part of the picture and it doesn't look nice.
To remove it grab the Brush Tool (B), select one of the round brushes and set the Hardness on 100%. Set the Master Diameter on 5 pixels. Higher the opacity of the brush on 100% too and pick black color.
Click on the layer mask, zoom in and paint over the cottage to make it invisible.