Start by opening the image you wish to alter. I chose to use the
background image from one of Adobe's tutorial images.



Create a new image with the same width as your image and a height of one pixel.
Fill this image with black



Now change the canvas size by clicking IMAGE>>CANVAS SIZE. Change
the height to 2 pixels while keeping the width the same size. Be sure to force the anchor upward.



You will now have an image with one pixel black and one pixel white. Create
a new selection by pressing Ctrl+A. Now you need to make this selection the active pattern by clicking EDIT>>DEFINE PATTERN.



Return to your backgound image and create a new layer. Fill this layer with your
new pattern by clicking EDIT>>FILL and choosing Pattern for the contents and the blending Mode as Overlay



There you have it. Now play around with the Mode settings and have fun.