1. Open up a new 5x5 pixel image with a transparent packground.

2. Select your Pencil Tool and set it to 1 pixel. Set your foreground color to black.

3. Now draw something like a plus sign on your image (You might have to zoom in on your image). It should look like this:



4. Define the pattern by going to Edit > Define Pattern. Now open up the image you are going to be applying the grid to.



5. Now grab your Paint Bucket Tool and in the options bar (Window > Options), change the Fill to Pattern. Next to Fill option you will see the Pattern Option. Find your pattern in there and then create a new layer.

6. Use your Paint Bucket Tool on the layer you just created. In the Layers Palette, adjust the opacity to lighten or darken the grid. Here is my final image: