In this tutorial you will learn how to make the square pattern you see on some images. This is only one way of doing it, in fact you could vary a few details in this tutorial till you arrive at your desired result.
Step 1: Create a new document with the following settings:
Width=5px
Height=5px
Background Contents: Transparent
EXTRA NOTE: I suggest this size as anything smaller and too many squares will be displayed. If you find that 5x5 is displaying too many squares, than try something like 7x7 or 9x9, the rest of the tutorial will still be the same.
Step 2: You'll probably notice that the document you just created is a too small to work with, to rectify this, zoom in, I went to 800%.
Using the PENCIL tool with size 1px make a square starting 2 pixels in from the edge of the image with black (#000000), use white if this the pattern is being applied to dark backgrounds.
Step 3: Go to EDIT->DEFINE PATTERN
You will see a window appear where you have to give your pattern a name. I called it Square, this makes it easier to find in photoshops library where it is saved.
Step 4: Click OK and open the image you want to add the squares to, I choose this Bugatti EB110 image. Or just make a selection if you don't want to apply the scan lines to the entire image. Create a new blank layer above the image and make sure that it remains selected (the layer should have a dark blue background).
Step 5: Now go to EDIT->FILL and select your pattern you just created, if you wish you can set the opacity here, as you can see I set it to 25%. I did it here as I already know how it looks at 100%. Click OK and there you go. If you didn't set the opacity in the FILL dialogue then you might find the squares too prominent so reduce the opacity and/or fill (in the layers palette) until your are satisfied with the result (go under 50 % for best results). The squares should be visible, but not overwhelming.
Here's how mine turned out.
