It is a relatively simple matter, once you have the idea and the base photographs. Firstly you have to prepare the two images as you would normally - adjust levels, contrast and so on until you are satisfied with the quality of the images. Today I chose two images related to Milner Field House, Titus Salt Junior's home a mile or so from Saltaire: a picture of the house itself and a photo of all that remains of it now - a pile of moss-covered stones.
I used the elliptical marquee tool to select an area in the middle of the stones image, feathered it by 150 pixels and decreased the brightness and contrast of the middle of the picture (so that it would not compete too much with the main part of the house image when overlaid.) I then dragged the house picture on top of the stones picture (use the Move tool) and sized it (by pulling the 'handles') to cover the stones image underneath completely. I adjusted the opacity of the (house) layer to about 60% (basically experimenting until it looked right) so that the mossy stones showed through. You can also try different blending modes at this stage to see what effect that has, though here I decided to leave it at 'normal'.The next step was to desaturate the image (in Channel Mixer - mono) and then add sepia toning by using the Colour Balance sliders. I then added the text and a stroke border.
The result is intended to hint at the mystery behind why a great house like Milner Field should have been reduced to rubble. Its story is well-known - and outlined on my other blog (see January 11 & 12 2010).
