Double Exposure

Open your portrait by dragging your photo to the Photoshop Icon

Use your Object Selection Tool to select your portrait

Once your selection has been made, go to Select - Select and Mask

Change your view mode to whatever you are comfortable using, I prefer using the Overlay mode, however any of them will work...

Use the Refine Edge tool to soften the edge of your selection

Once you're happy with your selection, click on OK. This will bring you back to your original photo with the new selection made.

Copy and Paste your selection to a new layer

Next, find the landscape you are going to use for the double exposure and drag it directly to your portrait photo

Hit Return to place your landscape to a new layer

Bring the Landscape layer below your Portrait Layer

Change the blend mode of the Portrait layer from Normal to Screen

Holding the Command key, click on the thumbnail of your Portrait layer select everything on that layer

Highlight your Landscape layer and click the Add Layer Mask button (bottom of layers window white rectangle with black circle)

DONE!