Making Grid for the Background

1. Image New.
2. Change the foreground color to your choice of grid color.
3. Using rectangular marquee, drag it to square or rectangle.
4. Edit > Stroke. Set pixel=1 or to the thickness of your choice.
5. Select > Modify > Border > pixel = 1.
6. Now save it. What you'll get is a square of 20x20 in pixels for the background (i.e. a single square). What I'm showing is the tiled image for easier viewing.