How to... produce a multiple image

 First of all you open up Photoshop on your computer/mac. As seen on the image above.

 Your screen should now appear as above. 

 Next step would be to open your background image that you will be using. For this you simply go to the top of your screen and file>open... (cmd and O shortcut)

Then the box should appear as image above shows, where you select your photograph and click open.

Your image should appear as above shows.

Next step would be to edit your image if needed. I have just adjusted the contrast of my picture by image>adjustments>levels...

This is the box that will appear where you can experiment with the mountain like box and see what fits your standards, then ok.

 After this the boxes to the right of your screen should be shown. You should have layers and history open which help you with the next few steps. (If these are not already on your screen, go to window>and click on which box you need)

You would next need to open another image that your going to use as you did before. To add this image onto your other as an over-lapping (layer) image, use the arrow and cross icon situated on the left of your page. Once doing that make sure your clicked on your new image you have just opened and while clicking drag it into your background photograph.

This image will now be named as 'Layer 1' in your layer box to the right. Next step would be to make your new photograph a layer mask. This is by clicking the icon at the bottom of the layer box. It is a grey box with a white circle in the middle. The 3rd icon in on the picture above.

 Now your Layer box should look like this. Make sure you are clicked on the layer mask before continuing.

Going to the left of your screen at the bottom you should see two boxes which are over-lapping. The one in the picture above are white and black. If you click one a box will come up as bellow.

This is the box that should appear, make sure your colours are pure - black would be all 0's and white would be 255. Press ok.

Now you go to the paintbrush icon, shown on the right hand column of your screen.

Lastly you paint where your background image would be. This should be when you use your black paint, which would make your image appear. 

Once you have added as many layers you want you go to Layers>flatten image, which will put all your layers into one. 

Then Save.