Photoshop - Creating a Layer Mask and Make a Montage

First you need a background, which provides a suitable backdrop for your Montage:

The background image size should be about 900 - 1000px wide. In this example I have an image at 960px wide.

Then in the Layers panel, New Layer.
Then you need an image to go over the top in a new layer.
This image needs to be in proportion to the background. This image is 634px wide. Not too big and not too small - in proportion.
In Photoshop, File Open and open the background image.
Then in the Layers panel, New Layer.
Then open the over the top image. The over the top image becomes layer 1.
The over the top image is now on top of the background. Also included is the over the top image's unwanted background.
It's to remove the unwanted sky!
Back to the Layers Panel and Add a Mask to Layer1.
Now it's time to erase the unwanted background. Using the Majic Wand Tool - select the area to remove and the Majic Wand removes parts.

The size is different only because a Screen Dump has been used to show the work in progress. But the 'real image' is the same - see below.
You may also use the Rectangular Marquee Tool.
Also the Eraser Tool to tidy up.
The final Montage should be clean and clear of unwanted background of the over the top image in Layer 1.
There's the Montage completed!
Notes: In selecting your Over the Top background - use an image with not too much background detail, as it could take you a long time to correct.

Image Sizing in Google Images: